There are

80

students in a speech contest. Yesterday,

1

4

of them gave their speeches. Today,

2

5

of the remaining students gave their speeches. How many students still haven't given their speeches?

Yesterday, 1/4 of 80 = 20 gave their speeches, so there are 80-20=60 students left.

Today, 2/5 of 60=24 students gave their speeches, so there are 60-24=36 students who still haven't give their speeches

another way to think of it is 80 * (1 - 1/4) * (1 - 2/5) = 80 * (3/4) * (3/5) = 36