If the mean (average) of five consecutive numbers is 21, the smallest of three five numbers will be ...

Consecutive numbers are 1 apart

they are

x, x+1, x+2, x+3, x+4

average is 21

(x+x+1+x+2+x+3+x+4) / 5=21

(5x+10) / 5=21

times both sides by 5

5x+10=105

minus 10 both sides

5x=95

divide both sides by 5

x=19

x+1=20

x+2=21

x+3=22

x+4=23

the smallest number is 19