Answer:

If 2/5 are girls then 3/5 are boys

These added together equal 5/5 which is the total number in the class ( 45 in this example )

45 divided by 5 equals 9

3 fifths would be 3 x 9 = 27 boys

The other way is to let x = number of boys

multiply 45 ( total # students) by (3/5)

x = 45 (3/5)

x = 135/5 = 27 boys