Great question! Well, a way to do this is using decimal points.
We know 75% is a whole number so if we thought of it as a regular number we could say 75.
In the number 75 our decimal point is here:
75. 
We can think of 75 like a normal number now. Well since 75(in the way we thought of it before) as a percent is out of 100, we can say
75/100. A fraction.
Now, we can narrow that down to 0.75! Why? Because let's think of it as a whole number.
75.
We need to move the decimal back two places because it is out of 100! Here's another example.
Ex. What is 40% as a decimal?
40/100= 40% (Percent means out of 100)
40 as a whole number= 40. (Decimal Point after the 0)
Move it back two times because it is out of 100. Why? Because there are 2 zeroes in 100(100)
So it is 0.40.
FOR YOUR MAIN QUESTION IT IS
0.75!
Hope this helps!