OK here is a really weird looking kinematics problem ...

A body is travelling at 20 m/s when it passes point P and 40 m/s when it passes point Q. Find its speed when it is halfway from P to Q, assuming uniform acceleration.

This is what me and my teacher did-

1. A velocity - time graph with initial point at (0,20) and it ends at v=40. I called that point (t2,40). Then the point when it is halfway through is (t1,v1). I tried to equate the area of 1st trapezium with the second and called that equation 1. Then is made another equation by equating the gradients of two segmets. But at any time I only have 2 equations with 3 unknowns!(Worried)

I know I'm missing a tiny thing nut can't figure it out...

thnx...