A bicyclist in the Tour de France crests a mountain pass as he moves at 15 km/h. At the bottom, 4.2 km farther, his speed is 65 km/h. Estimate his average acceleration while riding down the mountain.

18 km/hr*1000m/km*1 hr/3600 sec = 5 m/sec

85 km/hr = 23.6 m/s

distance = 4000m

distance = initial distance+initial velocity*time + 1/2 acceleration*time^2

initial distance=0

4000 = 5.0*t + 1/2 (accel) (time) ^2

Velocity=initial velocity + acceleration * time

23.6 m/s = 5.0 m/s + acceleration * time

We now have two equations and two unknowns. Solve them and you have your answer.