One pipe can fill a cistern in 1.5 hours while a second pipe can fill it in 2 and 1/3 hours.Three pipes working together fill the cistern in 42 minutes.How long would it take the third pipe alone to fill the tank?

Answer 1

using minutes for all pipes, we have
1/90 + 1/140 + 1/x = 1/42
x = 180
so, it will take the 3rd pipe 3 hours

Answer 2

rate for pipe 1 --- 1/(3/2) = 2/3
rate for pipe 2 --- 1/(7/3) = 3/7
rate for pipe 3 --- 1/x

combined rate = 2/3 + 3/7 + 1/x
= (14x + 9x + 21)/(21x) = 42/60 = 7/10

1/[ (14x + 9x + 21)/(21x) ] = 42/60 = 7/10
21x/(23+21) = 7/10
210x = 161x + 147
x = 147/49
= 3 hours