Parking in a students lot cost $1 for the first half hour and $1.25 for each hour thereafter. A partial hour is charge the same as a full hour. What is the longest time that a student can park in this lot for $7?

The longest time is ? (simplify your answer)

Okay so the answer is 4.5 hours but not sure how is 4.5 hours.. can someone help me.! thanks