Hello!
The answer is:
First employee: 3 gallons
Second employee: 3 1/3 gallons
Total:

Why?
From the statement we know:
First employee uses 3/4 gallon each our, for 4 hours the will use:

Second employee uses 1 2/3 (5/3) gallon every 2 hours, meaning 5/6 gallon each hour.

Remember, to sum (algebraic sum) fractions we should follow the next steps:
First: Multiply both fractions denominators by each other in order to get the denominator of the new fraction.
Second: Multiply the numerator of the first fraction by the denominator of the second fraction.
Third: Multiply the denominator of the first fraction by the denominator of the second fraction.
Fourth: Sum (algebraic sum) each result and divide it into the number obtained from the first step (denominator of the new fraction).
Example:

Have a nice day!