A racecar driver speeds around a circular track of radius 100 m at a velocity of 50 km/hr. The coefficient of friction us between his tires and the road is 0.20. It begins to rain, reducing us to 0.15. What must he change his velocity to in order to maintain his path without skidding off of the track? (use units of km/hr.).

When on the verge of skidding

M V^2/R = M g * (mu,s)

mu,s is the static coeffiecent of friction, 0.15 in this case. Cancel the M's and solve for V. It wil be in m/s if you use g = 9.8 m/s^2, so you will have to convert that V
to km/h