A large hose will fill a pool in 40 minutes while it takes a smaller hose 60 minutes to fill the same pool. The same pool requires 80 minutes to drain if the drain is open. Suppose one day both hoses are turned on, but by accident the drain was left open. How long would it take to fill the pool that day?

Question

I have tried the following
1/40+1/60-1/80=
1/40=1/60=1/x

Answer 1

I assume you are not a Calculus student...

rate water=water/minute
= pool/40min+pool/60min - pool/80min

=pool(1/40+1/60-1/80)
= pool (60+40-30)/2400=pool*(7/240)

now to fill the pool
is the inverse of this rate, or
240/7 min= about 34 min

Answer 2

this is just your classic work problem. You started out right, but then went off into the mists.

1/x = 1/40+1/60-1/80
1/x = 7/240
x = 240/7