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A particle P moves on a straight line with constant acceleration. At t=0, P passes through the point 0 on a line. When t=20, P passes through a point A, where OA=220m and when t=50, P passes through a point B where OB= 250m and AB=30m.

Calculate the initial velocity and acceleration of P.

This is what I did:

I spilt it into two equations, with my first being- 220=20u +200a and my second being- 250=50u + 1250a (using the suvat equation s= ut + 0.5at

^{2})

I then sub in u= 11-10a as u = 220-200a into my second equation to get 250= 550-500a+1250a, and therefore my a = -3/7ms

^{-2}

^{. }

In my answer book it says the the acceleration = -0.4ms

^{-2}, and so I gave up trying to find u... does anyone know where I went wrong ( or even is the book wrong- it can be very unreliable!!)

Thank you!!