Force is transferred from the moving ball to the stationary ball.
Explanation:
average speed = total distance travelled / total time travelled
time to travel the first 6km: 6 / 50 = 3/25 (h)
time to travel the next 6km: 6 / 90 = 1/15 (h)
[I think there's problem in the question 'cause 900km/h sounds impossible for normal person to travel in normal condition]
The total time: 3/25 + 1/15 = 14/75 (h)
Average speed over the 12 km drive will be:

Physical Change
characteristic is the chemical bonds in the substance are unchanged. Because
a physical change is any change happens in an object but without involving
a change in its chemical substance. Example, Solid to liquid change or
also known as melting, liquid to gas change also known as evaporation, gas
to solid change also known as deposition, liquid to solid or
solidification, solid to gas or sublimation, and gas to liquid or
condensation. The physical form of a substance is change into a new form
but the chemical is unchanged.