A school bus uses 34 of a tank of gas to drive back and forth to school in a week with 5 school days. How much gas does the bus use to drive back and forth to school in a week with 4 school days? Write your answer in simplest form.

I am assuming Michael's interpretation is correct, that you want to find how much gas is used in 4 days knowing how much is used in 5 days. Set up a simple proportion and solve it:

3/4 tank → 5 days

x tank → 4 days

x = 4 * (3/4) / 5 = 3/5 of a tank

In 4 days you use 3/5 of a tank.