Answer
20 hours
Work Step by Step
1st pipe: fills in 20 hours (1/20 done in an hour)
2nd pipe: fills in 15 hours (1/15 done in an hour)
$1/20 + 1/15 + 1/x = 1/6$
$6(1/20 + 1/15 + 1/x = 1/6)$
$6/20 + 6/15 + 6/x = 1$
$3/10 + 2/5 +6/x =1$
$3/10 + 4/10 + 6/x = 1$
$7/10 + 6/x = 1$
$6/x = 3/10$
$3x = 60$
$3x/3=60/3$
$x=20$