The united states gets about 39% of its energy from petroleum. express 39% as a decimal.

Percentages are easy to represent as decimals. Just remember this: If the value is under 100%, just place a decimal point in front of the percent value and remove the percentage sign. If it's over 100%, you'd put the decimal after whatever is in the hundreds place. The reason you can do this is because percent represents a value out of one hundred, and decimals do the same representing values as part of one.
So, 39%->.39