A 10 hr B 20 hr C 30 hr Exactly are pair of taps is open during each hour and every pair of taps is open at least for one hour. First A and B are open for 1 hour then B and C and then C and A
(
1
10
+
1
20
)
first
+
(
1
20
+
1
30
)
second
+
(
1
30
+
1
10
)
third
=
22
60
In three hours the tank will be filled (
22
60
)th part. Now, for minimum time the rest tank must be filled with A and B taps (
1
10
+
1
20
=
9
60
) So the rest (
38
60
)th part of tank will take 5 hours more. So the tank will be filled in 8th hour.