Susanna can work 10 math problems in 15 minutes. How many can she work in an hour and a half?

Well this one is easy!

10 ~ 15

20 ~ 30

30 ~ 45

40 ~ 60

50 ~ 75

60 ~ 90

90 = 1 hour and 30 minutes! < 60 math problems.

She can work 60 in an hour and a half. We find this by figuring out how many minutes are in an hour and a half (there are 90 minutes), dividing that number by 15 (which equals 6), and multiplying that number by 10 (which equals 60). 60 problems is the answer.