First convert the fraction so that it has a denominator of 100:
15/20 = (15 * 5)/(20 * 5)
Ignore the denominator and only look at the numerator (75). The decimal point in this number is after the 5. So it can be rewritten as 75.0
What you need to do here is move the decimal point two numbers to the LEFT. Meaning it'll come right before the 7 and the 5, like so:
.75 or 0.75
And that's the decimal form of 15/20.