Time Question

1. The problem statement, all variables and given/known data
A commuter train 500 m long travels on a straight track at a speed of 78.6 km/h it slows down as it approaches a crossing. The last car passes the crossing at a speed of 15.8 km/h. With a constant acceleration, how long did the train block the crossing?

2. Relevant equations
avg v=(v0+vf)/2
t=change x/avg v

3. The attempt at a solution
First I converted 78.6km/h*1000m/km*1h/3600 secs=78600/3600=21.83 m/s
Secondly I converted 15.8km/h*1000m/km*3600h/3600secs=15800/3600=4.39 m/s

Using avg v=21.83+4.39/2=13.11 m/s


I am pretty sure I did this correctly, but it took me a long time figuring it out (like what to do first and what equations to use). Any advice?


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