To a class of 28 pupils, 12 new pupils whose average age is 10 months less than the average age of the 28 are added. By how many months is the average age of the whole class lowered?
average age of original class : x months
average age of the 12 added : x-10
original class: total of ages in months = 28x
total in months of students added = 12(x-10)
new total of all students = 28x + 12(x-10) = 40x - 120
average of new class = (40x-120)/40 = x - 30
change in average = x-30 - x = -30
So the average age decreased by 30 months.