Would 15.75 be as a fraction 15/750

No, 0.75 is 3/4 and 15 is 60/4 therefor add them together to get 63/4, which can't be cancelled down. Therefor 15.75 is 63/4 as a fraction. It is not 15/7500 15 is smaller than 7500 therefor there would be no whole numbers. Remember a fraction is one thing dividing by another.

To convert decimals to fractions is easy

.75=75/100 or 3/4

so 15.75=63/4

also since 15/7500 is less than one, it cannot be equivilent to 15.75