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

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.

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.