Question

It takes a printer 9 hours to print the class schedule for all of the students in a college. A faster printer can do the job in 6 hours. How long will it take to do the if both printers are used?

Answer 1

Here you have to use a formula:

$\frac{t}{9} + \frac{t}{6}=1$

Once you solve, you get:

15t=54
t=54/15