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June

Boys -> 100u + 940

Girls -> 100u

July

Boys down to (100% – 15%) = 85% -> 0.85 x (100u + 940) = 85u + 799

Girls up by 35% -> 1.35 x 100u = 135u

Total -> 1943

 

85u + 799 + 135u = 1943

220u = 1144

u = 5.2

Boys in July -> 85u + 799 = (85 x 5.2) + 799 = 1241

0 Replies 2 Likes ✔Accepted Answer

In June :
boys ——- 1u + 940
girls ——- 1u

In July :
boys ——- 1u + 940 – 15% x (1u + 940) = 0.85u + 799
girls ——- 1u + 35% x 1u = 1.35u

1.35u + 0.85u + 799 ——- 1943
1u ——- (1943 – 799)/2.2 = 520
0.85u + 799 ——- 0.85 x 520 + 799 = 1241

Ans : 1241 boys.

By the way, I believe that you had asked the same question more than a year ago as per link below :  https://www.kiasuparents.sg/kiasu/forum/viewtopic.php?f=69&t=280&start=16540

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@siglapmaths

can we assume that 100%=10 units instead of 20units?

 

thanks to siglapmaths and Bigdevil

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Hi Ozora,

Just a note. There is a subtle difference in using LCM units and 100u (an easy and quick fix especially for question with %) and any other units or algebra. End results will be the same so it is up to individual preference, as long as the user’s mind is clear on what the unknown denotes.

However, based on my experience, if I use LCM units and get “u” as not a whole number, then I know immediately somewhere I have been careless and will stop and do a quick check. For 100u or some other “u”, it is possible to get a fraction or decimal answer for “u” and additional check is needed to put the “u” value back into the working to ensure accuracy. For this question, there is 135u and 85u involved and putting 5.2 into 135u and 85u gives whole number which makes sense for no. of girls and boys.

You can even don’t think about LCM units or 100u and just put a “G” for no. of girls and still can get the correct answer. Just that double checking is more necessary. See below where I picked from BigDevil’s workings (credit to BigDevil), but using “G” for no. of girls. (and this will look more like my old school algebra method.)

June

Boys -> G + 940

Girls -> G

July

Boys down to (100% – 15%) = 85% -> 0.85 x (G + 940) = 0.85G + 799

Girls up by 35% -> 1.35 x G = 1.35G

Total -> 1943

 

0.85G + 799 + 1.35G = 1943

2.2G = 1144

G = 520 [additional check 0.85G=442 is ok, 1.35G=702 is ok]

Boys in July -> 0.85G + 799 = (0.85 x 520) + 799 = 1241

Hope this puts your mind at ease from worrying about choosing the “correct” units. But I do suggest you try LCM units sometimes to get more practice and a good grasp of what LCM units represent and how they help in solving some unique questions. Do also have a play with 10u like you have asked earlier. You will still get same results. See another example here if you like.

https://www.kiasuparents.com/kiasu/question/32749-2/#post-32752

I also see some workings in your picture where child has written 15% = 3/20, 35% = 7/20. For these two fractions, the LCM units is 20u. Increase or decrease from 20u can easily be converted to increase or decrease by 3u or 7u, that is why the ease of use for LCM units. Same goes for 100u and % changes.

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@SAHMom

thanks for the tips. 

Appreciate that =)

0 Replies 0 Likes

Hi ozora

With 10 units, you will need to deal with fractions when you are solving for the unit value. With 20 units you will only have to deal with whole numbers.

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