There are a couple ways to find this. You could...
A. Since the question states "21 units on the first 3 floors" you could multiply 21 and 10 to get 210 (number of units if there were 21 on each floor), then divide it by 3 to find the total number in the building. Dividing by 3 compensates for the 21 units on the first 3 floors. Or...
B. Divide 21 by 3 to find how many units are on each floor (7), then multiply by the number of floors in the building (10). Either way, both ways will give you 70.
