Question 380404
Find the hourly rate of each event.
If the first pipe can fill the tank in 7 hours, then it can fill (+) {{{1/7}}} of the tank in 1 hour.
If the second pipe can empty the tank in 9 hours, then it can empty (-) {{{1/9}}} of the tank in 1 hour.
Putting these two events together, we get:
{{{1/7 - 1/9 = 2/63}}}
So {{{2/63}}} of the tank is filled in 1 hour.
It takes {{{63/2}}} hours to fill the tank.
{{{63/2 = 31}}}{{{1/2}}}hours.