# Some Math Questions and Answers

**Question:**

There were some boys and girls in a room.

If 24 boys leave the room, the total no. of students who remained in the room will be 7 times theno. of boys who remained the the room.

If 12 girls leave the room, the total no. of students who remained in the room will be 4 times the no. of boys.

How many students are there in the classroon?

**Answer:
**Boys at first = 1u + 24 = 1p ………… (1)

Girls at first = 6u = 3p + 12 ………… (2)

6u = 3(1u + 24) + 12

3u = 84

1u = 28

No. of students in classroom = 7u + 24 = 7(28 ) + 24 = 220

**Reasoning:**

If 24 boys leave the room:

At last

Boys : 1u

Girls : 6u

At first

Boys : 1u + 24

Girls : 6u

If 12 girls leave the room

At last

Boys : 1p

Girls : 3p

At first

Boys : 1p

Girls : 3p + 12

The number of boys and girls at first must be the same

1u + 24 = 1p ————– (1)

6u =3p + 12 ————— (2)

Solve to find the no. of boys and girls

**Question:**

1) 65% of the animals on a farm were cows and the rest were goats. When 240 more cows and goats were added to the farm, the percentage of cows increased by 20% and the number of goats doubled. How many goats were there on the farm at first?

**Answer:**

At first

Cows => 65% = 65u

Goats => 35% = 35u

At last

Cows => 65u + 13u = 78u

Goats => 35u + 35u = 70u

13u + 35u = 48u = 240

1u = 5

No. of goats at first = 35u = 35 x 5 = 175

**Question:**

2)There are some black and white marbles in a bag.If 20 black marbles are removed from the bag, the total number of marbles left will be 7 times the number of black marbles left. If 50 white marbles are removed from the bag, the total number of marbles left will be 5 times the number of black marbles left.How many marbles are there in the bag?

**Answer:
**If 20 black marbles removed

At lastBlack : 1u

White : 6u

At first

Black : 1u + 20

White : 6u

If 50 white marbles removed

At last

Black : 1p

White : 4p

At first

Black : 1p

White : 4p + 50

1u + 20 = 1p ————— (1)

6u = 4p + 50 —————-(2)

6u = 4( 1u + 20 ) + 50

6u = 4u + 130

2u = 130

1u = 65

No. of marbles in the bag = 7u + 20 = 7(65 ) + 20 = 475

## by the way im a student

by the way im a student

## i have a better answer for

i have a better answer for this question:

There are some black and white marbles in a bag………

so at first without any change:

B : W

1u+20 6u

After white is taken away,

B : W

1u+20 4u+80

Why 4u+ 80? coz 1u+20 x 4=4u+80.

ok so,

6u-(4u+80)=50

coz its actual no. of black marbles minus actual no. of black marbles taken away which is 50.

so

4u+80+50=6u

also means that

80+50=6u-4u = 130=2u

which means

1u=65

65x 7+20=475(original no. of marbles)

## Thanks for sharing :)

Thanks for sharing 🙂

## please look at your argument again...

Quote==>"After adding more naimals, the number of cows is (0.65+0.13)x(A+240), not (0.65+0.13)xA…"

after adding more animals, the number of cows is only 0.65+0.13 x A.you shouldn’t add 240 again, since that is the

totalnumber of animals added, with will include the 0.35A of goats and the 0.13A of cows (which is already added)…the "u" should remain the same through out the sum, since it is the base…

if you’re using "A", then the same goes:

240 = 0.35A + 0.13A = 0.48A

1A = 5.00….etc……

## quite difficult. thanks for

quite difficult. thanks for sharing

## Do not understand the solution for the question

For the following question, I do not understand why the unit is the same size after adding 240 animals as before adding them. Apparently, if we assume the number of animals at first is A, after adding 240 animals, the number is changed to 240+A. If percentage of cows is 65%, then the number of cows at first is 0.65A. After adding more naimals, the number of cows is (0.65+0.13)x(A+240), not (0.65+0.13)xA, which means that the base has increased. It may not be appropriate to assume the same "u" before and after the adding anaimals.———————–Question:1) 65% of the animals on a farm were cows and the rest were goats. When 240 more cows and goats were added to the farm, the percentage of cows increased by 20% and the number of goats doubled. How many goats were there on the farm at first?

Answer:At first

Cows => 65% = 65u

Goats => 35% = 35u

At last

Cows => 65u + 13u = 78u

Goats => 35u + 35u = 70u

13u + 35u = 48u = 240

1u = 5

No. of goats at first = 35u = 35 x 5 = 175

## P5 / P6 Standard.

P5 / P6 Standard.

## For the last two questions,

For the last two questions, it was taken out from a practice paper.If i’m not wrong,my school used ACS Primary Prelim Maths as a school practice paper and the last two questions appeared.

## Wow, which level are these

Wow, which level are these questions for? I was no where close when trying out.

## er which level are these

Happy 45th Birthday ! Singapore !!