If 100 cats kill 100 mice in 100 days, then 4 cats would kill 4 mice in how many days ?
a. 1 day
b. 4 days
c. 40 days
d. 100 days

If 100 cats killed 100 mice in 100 days, you can say that each cat killed one mouse the day after the last cat. So, if four cats killed four mice, and we're going of off the same basis that each cat killed one mouse the day after the last cat, then the answer would be 4.