Question

Cna i get some help with dis plz

Answer 1

Let $b_1,b_2,\ldots,b_{20}$ be the 20 marks of the boys, and $g_1,g_2,\ldots,g_{10}$ be the 10 marks of the girls.

We know that the global mean was 70, meaning that

$\dfrac{b_1+b_2+\ldots+b_{20}+g_1+g_2+\ldots+g_{10}}{30}=70$

Multiplying both sides by 30 we deduce that the sum of the scores of the whole classroom is

$b_1+b_2+\ldots+b_{20}+g_1+g_2+\ldots+g_{10}=2100$

By the same logic, we work with the marks of the boys alone: we know the average:

$\dfrac{b_1+b_2+\ldots+b_{20}}{20}=62$

And we deduce the sum of the marks for the boys:

$b_1+b_2+\ldots+b_{20}=1240$

Which implies that the sum of the marks of the girls is $2100-1240=860$

And finally, the mean for the girls alone is

$\dfrac{860}{10}=86$