Question 582103
One solar heating panel can raise the temperature of a quantity of water 1 degree in 30 minutes.
 A second solar heating panel can raise the temperature of the same quantity of water 1 degree in 45 minutes.
 How long would it take to raise the temperature of that quantity of water 1 degree with both solar panels 
:
Treat this as a "shared work" equation
:
let t = time required when both panels are used
let the completed job = 1 (raising a qty of water 1 degree)
:
{{{t/30}}} + {{{t/45}}} = 1
multiply by 90, to clear the denominators, results
3t + 2t = 90
5t = 90
t = 90/5
t = 18 min working together