3 pumps working 8 hours a day, can empty a tank in 2 days. How many hours a day must 4 pumps work to empty the tank in 1 day?

Let the required no of working hours per day be x.

More pumps , Less working hours per day  (Indirect Proportion)

Less days, More working hours per day      (Indirect Proportion)

  $Pumps4 : 3Days1 : 2⋮⋮ 8:x$

=> (4 * 1 * x) = (3 * 2 * 8)

=> x=12