Question: two pipes running together can fill swimming pool in 12 hours?
Answer: So the entire tank of 12 lit can be filled by second pipe alone in 12 hours. As both the pipes together can fill 3 lit in 1 hour, first pipe alone can fill 2 lit in 1 hr and thus can fill the entire tank in 6 hours. So individually pipe A require 16/3 hours and pipe B require 16 hours to fill the pool.