Question

Three science classes A, B and C take a life science test. The average score of class A is $83$. The average score of class B is $76$. The average score of class C is $85$. The average score of class A and B is $79$ and average score of class B and C is $81$. Then the average score of classes A, B and C is

Answer 1

$A:B=3:4$

$B:C=4:5$

$\Rightarrow$ $A:B:C=3:4:5$

$\Rightarrow$ Average $=\frac{3\left(83\right)+4\left(76\right)+5\left(85\right)}{3+4+5}$

$=\frac{249+304+425}{12}$

$=\frac{978}{12}$

$=81.5$