A boy in a wagon starts from a rest position and then goes down a straight ramp, gaining speed with uniform acceleration as he travels. In 6.00 s, he is travelling at 4.2 m/s. a) What is his velocity at 10.0 s? a = v2 – v1 / Δt a = 4.2 m/s [down] – 0 / 6.0 s a = 0.70 m/s^2 [down] v2 = v1+a Δt v2 = 0 + (0.70 m/s2) x 10.0 s v2 = 7.0 m/s [down] His velocity at 10.0 s is 7.0 m/s [down] b) How long will it take him to reach a speed of 28.0 m/s? This is where I am having trouble. I am unsure of the formula for calculating time. My book mentions Δt = v2 – v1 / Δt, but I believe this to be incorrect, so I tried Δt = v2 – v1 / a, but still could not get the answer they were looking for which is 40 seconds. Please advise.