第678题：Factors and Multiples

7 × 3 = 21, so 7 is a ?1 of 21, 21 is a ?2 of 7, and 7|21.

(Input the answer as multiple or factor.)

If you can divide a number A by a number B, without remainder, we say that B is a factor (or divisor) of A, and that A is a multiple of B. We often write B|A, where the vertical bar simply means “divides”.