Two identical cars hit each other and lock bumpers. In each of the following cases, what are the speeds of the cars immediately after coupling bumpers?
(a) a car moving with a speed of 90 km/h approaches a stationary car
(b) two cars approach each other with speeds of 90 km/h and 110 km/h, respectively
(c) two cars travel in the same direction with speeds of 90 km/h and 110 km/h, respectively