10. A pipe can fill a tank in x hours and another can empty it in y hours. They can together fill it in (y>x):

11. A cistern is normally filled in 8 hours but takes two hours longer to fill because of a leak in it sbottom. If the cistern is full, the leak will empty it in:

12. Pipes A and B can fill a tank in 10 hours and 15 hours respectively. Both together can fill it in: