68
google search
Google search results:
1: https://www.quora.com/Two-pipes-can-fill-a-cistern-separately-in-20-minutes-and-40-minutes-respectively-and-a-waste-pipe-can-drain-off-35-gallon-per-minute-if-all-three-pipes-are-opened-the-cistern-gets-fill-in-an-hour-What-is-the
++++++++++++++++
2: https://www.geeksforgeeks.org/pipes-and-cisterns-gq/
++++++++++++++++
3: https://gmatclub.com/forum/two-pipes-can-separately-fill-a-tank-in-20-hours-and-30-hours-291580.html
++++++++++++++++
4: https://books.google.com/books?id=I3rCgXwvffsC&pg=PA233&lpg=PA233&dq=Two+pipes+can+fill+a+cistern+in+20+minutes+and+30+m%0Arespectively.+Third+pipe+can+empty+the+tank+in+40+i%0Athree+pipes+are+opened+together,+how+long+it+will%0Atank+fuIl?&source=bl&ots=2m2t5wNKvH&sig=ACfU3U1p2LiBfj0DynB0aWjbyn27WJtCXw&hl=en&sa=X&ved=2ahUKEwjr1rWp39_yAhXXXM0KHSUjC3MQ6AF6BAgDEAM
++++++++++++++++
5: https://askoranswerme.com/28758/
++++++++++++++++
6: https://books.google.com/books?id=DO0jEAAAQBAJ&pg=RA1-PA88&lpg=RA1-PA88&dq=Two+pipes+can+fill+a+cistern+in+20+minutes+and+30+m%0Arespectively.+Third+pipe+can+empty+the+tank+in+40+i%0Athree+pipes+are+opened+together,+how+long+it+will%0Atank+fuIl?&source=bl&ots=N1x88-HpXl&sig=ACfU3U1_TFLm6U8vddtxVWgivOugr5LAng&hl=en&sa=X&ved=2ahUKEwjr1rWp39_yAhXXXM0KHSUjC3MQ6AF6BAgPEAM
++++++++++++++++
7: https://www.hitbullseye.com/Pipes-and-Cisterns-Questions.php
++++++++++++++++
8: https://books.google.com/books?id=5RZLDwAAQBAJ&pg=PA11&lpg=PA11&dq=Two+pipes+can+fill+a+cistern+in+20+minutes+and+30+m%0Arespectively.+Third+pipe+can+empty+the+tank+in+40+i%0Athree+pipes+are+opened+together,+how+long+it+will%0Atank+fuIl?&source=bl&ots=U8VFLDb8_b&sig=ACfU3U1Ij8FHt2GlimEoq97ZY4ktbdWHvA&hl=en&sa=X&ved=2ahUKEwjr1rWp39_yAhXXXM0KHSUjC3MQ6AF6BAgQEAM
++++++++++++++++
9: https://www.hitbullseye.com/Pipes-and-Cistern-Problems.php
++++++++++++++++
10: https://www.math-only-math.com/problems-on-pipes-and-water-tank.html
++++++++++++++++
11: https://www.careerbless.com/aptitude/qa/pipescistern.php
++++++++++++++++