Question

heavy car A of mass 2000kg traveling at 10m/s has a head-on collision with a sports car B of mass 500kg if both cars stop dead on colliding what was the velocity of car B

Answer 1

Applying conservation of momentum , 
P1=P2
Mass of car A = 2000 kg
Mass of car B = 500 kg
velocity of car B before impact =Vb
velocity of car B after impact =0
velocity of car A before impact=Va
velocity of car A after impact =0

2000 + Va+ 500 + Vb =0
2000 *10 +500 Vb =0
Vb= 20000/500 = -40
negative sign shows that direction are opposite

Answer 2

the answer is 40m/s.

Hope it is helpful

m1 (car A) =2000kg

m2(carB)=500 kg

v1 (car before impact) = va

v1(car after impact)=0

v2( car before impact)=vb

V2( car after impact) =0

2000×va+500×vb=0

2000×10+500vb=0

vb=20000/500=40m/s