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

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