Q:

Pipe A can fill a tank in 4 hours and pipe B can fill it in 6 hours. If they are opened on alternate hours and if pipe A is opened first then in how many hours, the tank shall be full? 

• 1
  $$3{1\over 2}\ hrs$$
• 2
  $$3{1\over 4}\ hrs$$
• 3
  $$4{1\over 2}\ hrs$$
• 4
  $$4{2\over 3}\ hrs$$