Written as a fraction, the amount is 17/20 .
But a fraction actually means "division". When you see "17/20", it means "17 divided by 20".
If you actually go ahead and DO the division problem (with a pencil or on your calculator) that the fraction is talking about, the answer you get is the same amount as the fraction, but written as a decimal.
The same amount as 17/20, written as a decimal, is 0.85 .
The same amount as 17/20, written as a percent, is 85% .