Three consecutive multiples of 3 are:
3n, 3(n+1), 3(n+2)
We are told that the sum of these multiples is 72 so:
3n+3(n+1)+3(n+2)=72 divide both sides of the equation by 3
n+n+1+n+2=24 combine like terms on the left side
3n+3=24 subtract 3 from both sides
3n=21 divide both sides by 3
n=7
Since the smallest number was 3n, the smallest number is 3(7)=21
So the smallest number is 21.
(the numbers are 21, 24, 27)