A car traveling 90 km/hr is 100 m behind a truck traveling 50 km/hr. How long will it take the car to reach the truck?

v = speed of car = 90 km/h

u = speed of truck = 50 km/h

d = initial separation distance = 100 m = 0.1 km

They meet at time t such that

vt = d + ut

t (v - u) = d

t = d / (v - u) = (0.1 km) / [ (90 - 50) km/h] = 0.0025 hours