SOLUTION: When 5 new girls joined a class, the percent of girls increased from 40% to 50%. What is the number of boys in the class

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Question 271570: When 5 new girls joined a class, the percent of girls increased from 40% to 50%. What is the number of boys in the class
Answer by ankor@dixie-net.com(22740) About Me  (Show Source):
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When 5 new girls joined a class, the percent of girls increased from 40% to 50%.
What is the number of boys in the class
:
originally
g%2F%28g%2Bb%29 = .4
g = .4(g+b)
g = .4g + .4b
g - .4g = .4b
.6g = .4b
multiply by 10
6g = 4b
:
When 5 girls are added
%28%28g%2B5%29%29%2F%28%28g%2Bb%2B5%29%29 = .6
g + 5 = .5(g+b+5)
g + 5 = .5g + .5b + 2.5
g - .5g = .5b + 2.5 - 5
.5g = .5b - 2.5
get rid of all those decimals, multiply by 2
g = b - 5
:
In the original equation, replace g with (b-5)
6(b-5) = 4b
6b - 30 = 4b
6b - 4b = 30
2b = 30
b = 15 boys
:
:
Check by finding the girls
originally
6g = 4(15)
6g = 60
g = 10 girls, a total class of 25
:
10/25 = .4, 40%
:
then we had 15 girls which is 50% of the class with 15 boys