Sexplain how knowing 50 times 4 =200 you find 500 times 400.

This is an easy way to solve multiplication with alot of zero's
Add only the numbers, then add the sum of the zeros
So with this problem we do this:-
5*4 is 20, plus the zero in the 50, so the answer is 200
Now we can do the other one:-

5*4 is 20, plus the 4 zeros that we removed is 200,000

Hope I helped. :)