Since 5 is simply 10 divided by 2, whenever you see 5, you can replace it with 10/2. And it is much easier to divide by 2 than multiple by 5, so multiplying by 5 simply equals to dividing by 2 and adding 0 at the end (or move the decimal point one place to the right).
For example,
48 X 5 = 48 X 10 /2 = (48 /2 ) X 10 = 24 0
Math Trick to fast multiply and Divide by 5., 25, 125 and 15