Question 663652
Working together, 2 water pumps A & B can fill a water tank in 3 hrs.
 Working alone pump A can fill the tank in 4 hrs.
 How long does it take pump B working alone to fill the same tank?
:
let b = B pump's time working alone
:
Let the completed job = 1 (a full tank)
:
A shared work equation, 
Each pump will do a fraction of job, the two fractions add up to 1:
:
{{{3/4}}} + {{{3/b}}} = 1
multiply by 4b to clear the denominators, results:
3b + 4(3) = 4b
3b + 12 = 4b
12 = 4b - 3b
b = 12 hrs B pump alone
:
:
Check that
3/4 + 3/12 = 1