Question 31095
Let x be the time:
Equation:
{{{1/20+1/15+1/x=1/6}}}
Multply the whole equation by 60:
{{{60(1/20)+60(1/15)+60(1/x)=60(1/6)}}}
{{{3+4+60/x=10}}}
3x+4x+60=10x
7x+60=10x
3x=60
x=20


Hence, it would take the pipe 20 hours to do it alone.
Paul.