A pipe of 2 inch diameter fills a water tank in one hour. If the diameter of the pipe is 4 inch in what time the pipe fill the same tank?    

For the first pipe, the area is: Pi * 22 = Pi * 4

For the second pipe, the area is: Pi * 42 = Pi * 16

Now,

pipe with Pi * 4 sq. inch area can fill the tank in 1 hour or 60 min

pipe with Pi * 16 sq. inch area can fill the tank in 1 hour or (60 * Pi * 4) / (Pi * 16) min

= (60 / 4) min

= 15 minutes