Four pipes can fill a tank in 70 minutes. how long will it take to fill the tank with 7 pipes are used

Set it up like a ratio: pipes on the top and minutes on the bottom. Therefore, your ratio would be 4/70 = 7/x. Cross-multiply to get 4x=490 and x = 122.5 minutes. Change that to hours if you'd like by dividing by 60!