If the first hill is 20 cm high, the second hill is 0 cm high, and the last hill is 0 cm high, how long does it take the 50 g car to reach the egg?

The answer is undefined. 

To get how long it takes the 50 g car to reach the egg, the speed is required. Distance is provided from the height of the hills. 

Formula to get the time is:

T = D/S
Time = Distance / Speed

For example, given that the distance is 20 cm and the speed of the 50 g car is 2 cm/s

T = 20 cm / 2 cm/s
T = 10 s

The 50 g car will take only 10 seconds to travel.