in solution
12 pupils is multiplied by 10 so 12X10=120
let the pupils who will get low grades as number x so
if there are 9 students got high grades its a simple equation, 9+X=12 then using the subtraction property of equality you can 9-9+x=12-9 to eliminate 9 from x so now you have x=3, since we multiplied 12 to 10 to get 120 so x=3(10) and the answer is 30. :)
