Question

How can 400x20 help solve 399x20

Answer 1

Just subtract 20 from the answer of 400x20. One number less.

Answer 2

What you can do is multiply 400*20= 8,000. From here, we know that 399*20 is just one less group of 20 than 400. Therefore, we can subtract (1*20) from 8,000 to get the final answer.

8000-20=7980

Hope this helps!