We need to write the following expression and solve for x:
9/18 = 100/x
So: 9x = 1800
x = 200
So it would take him 200 minutes to deliver 100 papers.
We do the same for 72 papers
9/18 = 72/x
so 9x = 1296
and x = 144
It would take him 144 minutes to deliver 72 papers.
