Question

17 out of 20 adults surveyed said they owned a cell phone. Represent the ratio 17 out of 20 as a percent.

Answer 1

Judging by the question I assume the best way to go about obtaining the percent would be to divide 17 by 20 in a calculator.

The answer you should get is 0.85.
You then want to multiply this product by 100, which this will get you your percent. 
The percent you should get is 85%

Answer 2

Divide the number of adults that own a cellphone by the total adults surveyed:

$17 \div 20 = 0.85$

Multiply the decimal by 100 to convert into a percentage:

$0.85 \times 100 = 85$

85% of adults surveyed have a cell phone.