Question

For 7/20 what is an equivalent fraction with a denominator of 100

Answer 1

100/20=5
7×5=35
35/100

Answer 2

Start with  7/20 .

We can multiply the denominator by anything we want to,
but if we do that, we must immediately multiply the numerator
by the same thing, to avoid changing the value of the fraction. 
(I guess you could call this process "un-simplifying" the fraction.)

The denominator is  20  right now, and we want it to be  100 .
So we have to multiply it by  5 .

Immediately, before we forget, we must also multiply the numerator by  5 .

So we end up with  35/100 .

This fraction has exactly the same value as  7/20 .
You should prove that for yourself by simplifying  35/100 .