Answer:
20 books
Explanation:
As a thought experiment, let's turn this around a bit:
What if you were asked to STACK 3" thick books, could you figure out how many you'd need to get to 5' high?
- Put one down: 3"
- Put a 2nd one on it: 6"
- 3rd: 9"
- 4th: 12"
Let's stop there, because 12" = 1', right? How many books to make a foot? 4
So how many books to make 5'? 20, right? 4 books per foot, times 5'. Or:
4 + 4 + 4 + 4 + 4 = 5(4) = 20 books
5 groups of 4 books that are 3" thick makes a stack 5' tall.
I just wanted you to think about what was physically going on before getting to "the math."
It's just a division problem, right?
How many of these things fit inside this bigger thing?
Where you may've gotten hung up was in the units, because inches don't match feet.
But that's simple, just convert the feet to inches:
5 ft x 12 inches/ft = 60 inches
See how "feet" cancelled out because it was in the top and bottom?
So the revised question is: how many 3" books makes a 60" stack, and the math for that is:
60 inches ÷ 3" = x
x = 60/3 = 20 books