Question 33703
water in - water out = 1pool
in: 10 hours  1/10  hours together -->x   part done is hours x/10
out: 20 hours  1/20  hours together -->x Part done is hours x/20
Equation:
{{{x/10-x/20=1}}}
Multiply the whole equation by 20:
{{{20(x/10)-20(x/20)=20(1/1)}}}
{{{2x-x=20}}}
x=20
Hence, it will take 20 hours to fill the pool.
Paul.