Three points A, B and C are on a horizontal line such that AB = 70m and BC = 35m. The angles of elevation of the top of the tower are x, y and z, where tan x = 1/13, tan y = 1/15, and tan z = 1/20. The foot of the tower is at the same level as A, B and C. Find the height of the tower.

I can draw the diagram and have 3 equations in 2 unkowns, can't go any further. Please help