21 × 17 =

Math multiplication is repeated addition, if we have 3 multiplied by 4, 3 X 4 = 3 + 3 + 3 + 3, this can be written as a sum where 3 is added 4 times.

2 × 1 = 2

5 × 2 = 10

3 × 3 = 9

By learning math, you learn to think.