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Question 547925: My question:
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?
What I tried:
1.If there is 50% girls in the class, then there is 50% boys, so however many girls are in the class, is how many boys are in the class. 2.I said that the ORIGINAL total class accumulation, was 45 kids, because if there is 25 boys in the class, and 20 girls in the class, the boy population is 60%, and the girl's is 40%. However, I said that if I increase the girl population by 10%, I would need to add five girls, so now there is 25 girls, and 25 boys (50% boys and girls) What am I doing wrong? Can you help me solve it? 3. My teacher said this was incorrect, and I got a 5/10 on this problem; 3 for work, 1 for solution, and 1 for check. If you could help me, I would appreciate it! Thanks a lot!!!
Answer by ankor@dixie-net.com(22740)   (Show Source):
You can put this solution on YOUR website! 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
 = .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
 = .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
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