To solve this problem, all we have to do is to closely analyze the situation and use the principle of ratio and proportion.
What we have to do first is to write the given values in terms or units of job / (pipes hour). In this case, the job refers to the action of completely filling the tank given the specified number of pipes and number of minutes.
For the 1st case:
1 tank / (4 pipes * 70 minutes)
For the 2nd case:
1 tank / (7 pipes * t)
Now to solve for t, we equate the two cases:
1 tank / (4 pipes * 70 minutes) = 1 tank / (7 pipes * t)
t = (1 tank / 7 pipes) / [1 tank / (4 pipes * 70 minutes)]
t = (1 / 7) / [1 / 280]
t = 40 minutes
Therefore it requires 40 minutes for 7 pipes to completely fill the tank.