SOLUTION: The average age of all the students of a class is 18.0 years. The average age of boys of the class is 20 years and that of the girls is 15 years. If the number of girls in the clas

Algebra ->  Average -> SOLUTION: The average age of all the students of a class is 18.0 years. The average age of boys of the class is 20 years and that of the girls is 15 years. If the number of girls in the clas      Log On


   

Question 324943: The average age of all the students of a class is 18.0 years. The average age of boys of the class is 20 years and that of the girls is 15 years. If the number of girls in the class is 20, then find the number of boys in the class?
Answer by Fombitz(32388) About Me  (Show Source):
You can put this solution on YOUR website!
Let B be the sum of the boys' ages, G the sum of the girls' ages.
Let Ng be the number of girls, Nb the number of boys.
.
.
.
1.%28B%2BG%29%2F%28Ng%2BNb%29=18
2.B%2FNb=20
3.G%2FNg=15
4.Ng=20
From eq. 3 and 4,
G=300
From eq. 2,
B=20Nb
Substitute into eq. 1,
%2820Nb%2B300%29%2F%2820%2BNb%29=18
20Nb%2B300=18%2820%2BNb%29
20Nb%2B300=360%2B18Nb
2Nb=60
highlight%28Nb=30%29