Answer:
12 hours
Explanation:
Make a proportion
He made 216 for 18 hours of work, and 144 for x hours of work
216/18=144/x
Cross multiply
216*x=18*144
216x=2592
We want to solve for x, and to do this, we need to get by itself. It is being multiplied by 216. To reverse this, do the opposite of multiplication: division.
Divide both sides by 216
216x/216=2592/216
x=12
He will need to work 12 hours