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

The faster car behind is catching up/closing the gap/gaining on

the slow truck in front at the rate of (90 - 50) = 40 km/hr.

At that rate, it takes (100 m) / (40,000 m/hr) = 1/400 of an hour

to reach the truck.

(1/400 hour) x (3,600 seconds/hour) = 3600/400 = 9 seconds, exactly

Since both these cars are traveling as an assumed constant rate, you can imagine them as not moving at all to make the problem easier conceptually. The distance between the cars will also remain constant. First, convert the units km/hr into m/s: 90/3.6 = 25 m/s; 50/3.6=13.89 m/s. The difference between these two speeds is 11.11 m/s. Now use the equation: velocity = distance/time. 11.11 = 100/time; solve for time and you should get 9.0009 seconds