Ok the sets are natural numbers (or counting numbers), this is like 1,2,3,4,5 etc
whole numbers, this is including 0, so 0,1,2,3,4,5,6 etc
integers, this includes the previoius set and negatives, so -3,-2,-1,0,1,2,3,4,5 etc
rational numbers, this is the set of numbers that can be written in form a/b where b≠0, so all integers can be written like this, like example -3=-3/1, so -7/9 belongs here
How many times must he see 9 in order to cover all 72 of them ? 72/9 = 8 . He has to see 9 patients 8 times, so he has to work 3 hours 8 times. 3x8=24 hours.