Two pipes that can fill a cistern in 5 hours and 8 hours respectively were used to fill two same-sized cisterns. How much time will it take to fill both the cisterns?

Question: Two pipes that can fill a cistern in 5 hours and 8 hours respectively were used to fill two same-sized cisterns. How much time will it take to fill both the cisterns?

Solution:
in one hour,
1st pipe will fill = 1/5
2nd pipe will fill = 1/8

total fill up
= 1/5 + 1/8
= 13/40

so, the total time to fill two cisterns = 80/13 hours
= 6.15 hours