Find the sum of all the multiples of 3 from 1 through 300. Do you see a pattern developing?
Find the sum of the multiples of 7 from 1 through 700. Have you found the pattern? Next suppose we let N be any whole number.
Find the sum of all the multiples of N from 1 through 100*N.
(Source: Adapted from Mathematics Teaching in the Middle School, Sep-Oct, 1994)