Answer:
7.4 km/h
Step-by-step explanation:
Since this involves momentum conservation, apply the principle of linear momentum.
The principle of linear momentum states that:
+
= (
+
)V
where:
is the mass of the man,
is the velocity of the man,
is the mass of the car,
is the velocity of the car, and V is the common velocity of the man and car.
But, since the car was initially at rest,
= 0.
70 x 18 + 100 x 0 = (70 + 100)V
1260 = 170V
V = 
= 7.4118
V = 7.4 km/h
The car and man would start to move with a velocity of 7.4 km/h.