What is the mean (average) of all counting numbers less than 100 that are multiples of 3?

Question

I would like to know the formula pls.

Answer 1

There are 33 multiples of 3 that are less than 100. Indeed, all multiples of 3 have the form 3k for some integer k. Their sequence is 3, (3)(2), (3)(3), ... , 3k, 3(k+1), ... ,3(33)=99<100, where 1<=k<=33. Their sum equals 3(1+2+3+4+...+33)=3(33)(34)/2=1683. Hence, the average is 1683/33=51.