How to find time difference?

Academic support for Primary 3

How to find time difference?

Postby tony chua » Thu Nov 12, 2015 2:59 pm

My girl (Jurong West Primary School) , P3 has just received back her SA2 maths paper and was surprised to see her teachers gave her wrong for the 2 time differences questions as per below .
1. Anna left home at 6.45am and arrived in school at 7.25am.
How many minutes did she take to reach her school?
(She took 7.25 minus 6.45 an got an answer of 40 min)
However, her teacher gave her wrong and wrote “wrong expression” in the paper. Teacher even told her there were no such way in doing time!

2. Huimin started reading a book at 8.30am.
She took 1 h 42 min to finish reading the book.
What time did she finish reading the book?
(She took 8.30 + 1.42 and got an answer of 9.72, converted into 10.12am)
Again, the teacher gave her wrong and wrote “wrong expression” and “fluke” next to her answer!

I like to ask all parents and hopefully there are teacher here – why she got wrong for above? why the teacher must force pupils to use the time-line method?

tony chua
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Re: How to find time difference?

Postby dancer2000 » Thu Nov 12, 2015 3:05 pm

you cant add time as it is mathematically wrong :) try to use arrows instead of addition or subtraction signs! and write the time "added" on top of the arrow.
for hours and mins, i recommend u comverting everything to mins :)

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Re: How to find time difference?

Postby Sun_2010 » Thu Nov 12, 2015 5:01 pm

tony chua wrote:My girl (Jurong West Primary School) , P3 has just received back her SA2 maths paper and was surprised to see her teachers gave her wrong for the 2 time differences questions as per below .
1. Anna left home at 6.45am and arrived in school at 7.25am.
How many minutes did she take to reach her school?
(She took 7.25 minus 6.45 an got an answer of 40 min)
However, her teacher gave her wrong and wrote “wrong expression” in the paper. Teacher even told her there were no such way in doing time!

2. Huimin started reading a book at 8.30am.
She took 1 h 42 min to finish reading the book.
What time did she finish reading the book?
(She took 8.30 + 1.42 and got an answer of 9.72, converted into 10.12am)
Again, the teacher gave her wrong and wrote “wrong expression” and “fluke” next to her answer!

I like to ask all parents and hopefully there are teacher here – why she got wrong for above? why the teacher must force pupils to use the time-line method?


Units play an important role in maths. It is a key concept and pays to thoroughly understand it.

I have $8. i gave away $3. How much do i have left?
8 - 3 = 5 correct
$8 - $3 = $5 correct
8 - 3 = $5 wrong

The units in the left hand side and right hand side should be same or convertable
Same way
7.25 - 6.45 = 40 mins is wrong.
7:25 am is not 7.25

Similarly 1hr 42 mins is not 1.42 hrs. Actually it is 1.7hrs

However I disagree with the teacher that the answers are a fluke. Your DD is quite strong in understanding concept of time and manipulating it. She just needs to represent it correctly.

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Re: How to find time difference?

Postby cyberette » Thu Nov 12, 2015 6:42 pm

tony chua wrote:My girl (Jurong West Primary School) , P3 has just received back her SA2 maths paper and was surprised to see her teachers gave her wrong for the 2 time differences questions as per below .
1. Anna left home at 6.45am and arrived in school at 7.25am.
How many minutes did she take to reach her school?
(She took 7.25 minus 6.45 an got an answer of 40 min)
However, her teacher gave her wrong and wrote “wrong expression” in the paper. Teacher even told her there were no such way in doing time!

2. Huimin started reading a book at 8.30am.
She took 1 h 42 min to finish reading the book.
What time did she finish reading the book?
(She took 8.30 + 1.42 and got an answer of 9.72, converted into 10.12am)
Again, the teacher gave her wrong and wrote “wrong expression” and “fluke” next to her answer!

I like to ask all parents and hopefully there are teacher here – why she got wrong for above? why the teacher must force pupils to use the time-line method?


Didn't the teacher teach them how to draw timeline this year? The teacher should've provided the workings when she returned the paper to your DD.

I agree with the teacher that your DD's method was fundamentally wrong. Granted she exercised her intelligence by converting the "72" to "60" + "12" mins. But when the questions get trickier as she advances to the upper levels, and if she still sticks to this fundamentally wrong method of add/sub, she will be lost and giving wrong answers. Let me explain why:

If Anna left home at 6.45am and came back at 7.25pm, your DD by using her add/sub method will get 40min. But we know it's not correct, yah?

If the end-time crosses the 12noon time mark, she will again be stumped as the end-time is now smaller than the start-time. E.g. Anna left home at 6.45am and arrived at 2.25pm. If she adheres to the same wrong logic, she will use "2.25 - 6.45" and derive "-4.20". She will stare at this negative figure, wondering if it means 4h20min or something that occured in the past! :yikes:

For Q2, if it became Huimin had flown off at 8.30am and took 5h55min to arrive at City A, your DD will happily write "8.30 + 5.55 = 13.85". If she's taught how to read the 24h clock and break "85" into "60+25", all's fine. If she wasn't, she'll be stumped.

But time gets more complicated as one moves up the ladder...Suppose Huimin had flown off at 12.45pm and took 5h55min to arrive at City B, which your DD may then write in decimals "12.45 + 5.55" to get a strange "18". This is incorrect, because Huimin would arrive at 6.40pm, not "18" nor "1800h".

Converting 1h42min to minutes might leave her lost too. E.g. she will write decimals "8.30 + 102" and get a weird figure "110.3", unless she remembers to add "102" to the "30" of "8.30" and do the necessary conversion! :sweat:

So you must remind her time and again that time isn't equal to decimals, like what Sun2010 said. Second, make her draw the timeline + leaping arrows and do what dancer2000 advised.

Q1: Draw a horizontal timeline and make a marking at 6.45am. Next, mark the upcoming hour 7.00am, so that she knows 6.45am -- 7.00am takes 15min. Then continue on the timeline till 7.25am which accounts for the next 25min. Make her add 15 + 25min = 40min.

Q2: Start the timeline from 8.30am and arrow leaps to the first hour 9.30am (1h). Then continue drawing the line for another 30min to the next hour 10am. Then the remaining 12 min will be marked as 10.12am on the timeline.


http://www.bbc.co.uk/bitesize/standard/maths_i/measure/time_calc/revision/1/

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Re: How to find time difference?

Postby Nebbermind » Thu Nov 12, 2015 6:51 pm

But why should we penalise a kid who is competent in the time concept?

Is the time line working compulsory even if one can do without?

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Re: How to find time difference?

Postby cyberette » Thu Nov 12, 2015 6:58 pm

I'm sure one can do without the timeline if one's able to visualise the clock and add/sub carefully with the clock in his mind :-) :rotflmao:

Penalise or not, we have to ask the teacher then...just like many adults wondered why this teacher marked the student wrong in this question that went viral last month.

http://www.patheos.com/blogs/friendlyatheist/2015/10/21/why-would-a-math-teacher-punish-a-child-for-saying-5-x-3-15/

Apparently, it's conceptually wrong to the mathematician. The first number represents the group/set in a multiplication expression, or so I learnt from the teacher! :sweat:

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Re: How to find time difference?

Postby slmkhoo » Thu Nov 12, 2015 7:38 pm

Nebbermind wrote:But why should we penalise a kid who is competent in the time concept?

Is the time line working compulsory even if one can do without?

I think the student should have got some marks as she obviously understood the question. But she didn't present the working method correctly, and mathematically, what she wrote did not make sense. It seems a little thing because it's still primary school and a common concept, but it's important to teach kids how to present mathematics correctly as it will affect their working as they get to more complicated concepts later on.

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Re: How to find time difference?

Postby Nebbermind » Thu Nov 12, 2015 7:45 pm

Admittedly, the 'working' is wrongly represented. Hopefully she got all her marks coz i remember math usually provides box or line where they are to insert the answer.

Is there any other way to represent time difference other than timeline?

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Re: How to find time difference?

Postby PiggyLalala » Thu Nov 12, 2015 11:54 pm

Nebbermind wrote:Admittedly, the 'working' is wrongly represented. Hopefully she got all her marks coz i remember math usually provides box or line where they are to insert the answer.

Is there any other way to represent time difference other than timeline?


What about using the 24hr clock? Is it allowed if the child used the 24hr clock notation: 0725 -0645 =0040. Then she wrote the answer as 40mins.

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Re: How to find time difference?

Postby slmkhoo » Fri Nov 13, 2015 7:25 am

PiggyLalala wrote:What about using the 24hr clock? Is it allowed if the child used the 24hr clock notation: 0725 -0645 =0040. Then she wrote the answer as 40mins.

Mathematically, the statement isn't correct. Although the student can mentally say "this is time, not a subtraction statement", it is bad practice and will cause problems when the maths gets more complex. At pr school level, a lot of the teaching is about maths basics like that, not just getting the answer.

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