Question 547925
When five new girls joined a class, the percent of girls increased from 40% to 50%.
 What is the number of boys in the class?
:
Use algebra to solve this:
:
Let g = original number of girls in the class
Let b = no. of boys
then
(g+b) = original number in the class
:
Originally
{{{g/(g+b)}}} = .4, (40%)
g = .4(g+b)
g = .4g + .4b
g - .4g = .4b
.6g = .4b
multiply 10 to get rid of the decimal
6g = 4b
:
Add 5 girls equation
{{{(g+5)/(g+b+5)}}} = .5
g + 5 = .5(g+b+5)
g + 5 = .5g + .5b + 2.5
g = .5g + .5b + 2.5 - 5
g - .5g = .5b - 2.5
.5g = .5b - 2.5
get rid of the decimal, mult by 2
g = b - 5
replace g with (b-5) in the 1st equation (6g = 4b)
6(b-5) = 4b
6b - 30 = 4b
6b - 4b = 30
2b = 30
b = 15 boys in the class 
:
:
Check this:
g = b - 5
g = 15 - 5
g = 10 girls  originally
then when 5 girls are added, it will be 15:15