A cistern can be filled by one tap in 8 hours, and by another in 4 hours. How long will it take to fill the cistern if both taps are opened together?
Given,
A cistern can filled by one tap = 8 hours
Another can filled it in = 4 hours
∵ Cistern filled by first tap in 1 hour = 
Cistern filled by another tap in 1 hour = 
part
∴ Net cistern filled in 1 hour = 
Thus, both cistern can fill the cistern in = 
hours
24