Jomar decides that he needs to study for an average of 1 hour and 30 minutes
a day in order to prepare for his French test this Friday. He studies for 2 hours
each day on Saturday through Monday, but is too busy to study on Tuesday
and Wednesday. How long must Jomar study on Thursday to be prepared for
his test?

If we want to make the average a certain amount of time over a certain number of days, we can first figure out the number of days and total amount needed. Here, we can see that the average is 1 hour and 30 minutes, and there are 6 days (Saturday-Thursday). Therefore, as

average = total / length, we can multiply both sides by length to get

average * length = total

Plugging 1.5 hours in for average and 6 days for length, we get

1.5 * 6 = 9 hours as the total amount needed to get an average of 1.5 hours a day

Our total so far is 2 hours for Saturday, 2 hours for Sunday, and 2 hours for Monday. This adds up to 6 hours, and we need 9. The difference between 9 and 6 is 3, meaning that Jomar needs to study for 3 hours on Thursday to be prepared for his test.