Summation? I don't know why you think it is correct. It may be flawed because summation is based on

being an integer only.

A fast way to do this question, however, would be to draw a velocity-time graph, and then realise that the time taken to reach 35 m/s is

seconds. Then, you can now use symmetry and say that in

seconds, the total displacement between the car and motorbike will be zero, so that the distance covered will be