IBPS Clerk Prelims Model Paper 21

There are 3 taps for three pipes x, y and z. x and y are inlet pipes, which can fill a tank in 20 hours and 25 hours, respectively, whereas z is an outlet tap. When the tank is empty, a man wants to turn on x and y, but by mistake, he turned on x and z. It takes 10 hours more to fill the tank than expected. In how much time can pipe z empty the tank alone?