A group of boys scored an average of 58 points in a game. When 2 girls each scoring 64 points and 70 points respectively joined the group of boys, the average became 61. How many boys were there? Ans : ___

1. Express the total score of the group in terms of the number of boys

Let the number of boys be $n$.
Total score of the boys = $58n$
Total score after the 2 girls join = $58n + 64 + 70 = 58n + 134$

2. Form an equation using the new average and solve for $n$

The new total number of people is $n + 2$.
The new average is $61$.
$\frac{58n + 134}{n + 2} = 61$
$58n + 134 = 61(n + 2)$
$58n + 134 = 61n + 122$
$134 - 122 = 61n - 58n$
$12 = 3n$
$n = 4$