Need finding out the recursive formula? ! really stuck on this problem.

How many kilograms of a 5% salt solution and how many kilograms of a 15% salt solution must be mixed together to make 45kg of an 8% salt solution?

Tom can paint the fence in 12 hours, but if he works together with a friend they can finish the job in 8 hours. how long would it take for his friend to paint this fence alone?

21 a stick 7 inches long is broken into two pieces, so that one piece is twice as long as the other one. how long are the two pieces?