12 Pipes All Of Same Size Fill Tank In 42 Minutes

12 pipes, all of the same size, fill a tank in 42 minutes. How long will it take to fill the same tank, if 21 pipes of the same size are…

Answers:

Pipes : Time :: Pipes : Time

12      :   2x     :   21      :   42

∴ By inverse proportion

12 × 42 = 21 × x

⇒ x = (12 × 42)/12 = 24 minutes