Question

Of a group of boys and girls at Central Middle School’s after-school party, 15 girls left early to play in a volleyball game. The ratio of boys to girls then remaining was 2 to 1. Later, 45 boys left for a football game. The ratio of girls to boys was then 5 to 1. How many students attended the party?

Answer 1

let b = original number of boys

let g = original number of girls

:

Of a group of boys and girls at Central Middle School's after-school party, 15 girls left early to play in a volleyball game.

The ratio of boys to girls then remaining was 2 to 1.

b%2F%28%28g-15%29%29 = 2%2F1

Cross multiply

b = 2(g-15)

b = 2g - 30

:

Later, 45 boys left for a football game. The ratio of girls to boys was then 5 to 1.

%28%28g-15%29%29%2F%28%28b-45%29%29 = 5%2F1

cross multiply

g - 15 = 5(b-45)

g - 15 = 5b - 225

g = 5b - 225 + 15

g = 5b - 210

Replace b with (2g-30)

g = 5(2g-30) - 210

g = 10g - 150 - 210

g = 10g - 360

360 = 10g - g

360 = 9g

g = 360/9

g = 40 girls originally

find b

b = 2(40) - 30

b = 50 boys originally

" How many students attended the party?"

40 + 50 = 90 students

Answer 2

90

Explanation:

Let g and b represent the number of girls and boys attending, respectively.

After 15 girls left, the ratio of boys to girls was ...

b/(g -15) = 2/1

b = 2g -30 . . . . . multiply by (g-15)

__

After 45 boys left, the ratio of girls to boys was ...

(g -15)/(b -45) = 5/1

g -15 = 5b -225 . . . . . . multiply by (b-45)

g = 5b -210

Using the latter to substitute into the former, we have ...

b = 2(5b -210) -30

450 = 9b . . . . . add 450-b

50 = b . . . . . . 50 boys attended

g = 5(50) -210 = 40 . . . . . 40 girls attended

The number of students who attended the party was 50 +40 = 90.

_____

Check

After 15 girls left, the ratio of boys to girls was 50/25 = 2/1.

After 45 boys left, the ratio of girls to boys was 25/5 = 5/1.