Here in this type of collision problem we can say that there is no external force on the system of two cars
So we can use the momentum conservation in this
![m_1v_(1i) + m_2v_(2i) = m_1v_(1f) + m_2v_(2f)](https://img.qammunity.org/2020/formulas/physics/middle-school/2zepqgg1yom87wia3810tpdoykerllulpd.png)
here we have
![m_1 = m_2 = 250 kg](https://img.qammunity.org/2020/formulas/physics/middle-school/6nx8t5chbhlx97mwcbpi60hw1z8ovkjb5r.png)
![v_(1i) = 4 m/s](https://img.qammunity.org/2020/formulas/physics/middle-school/uzf1gvj97z4ufi40lou2efyjjd7qm6a3pm.png)
![v_(2i) = 0](https://img.qammunity.org/2020/formulas/physics/middle-school/h062zabr4198ffjir88gc2o3sfwtmzwdq2.png)
![v_(1f) = v_(2f) = v](https://img.qammunity.org/2020/formulas/physics/middle-school/ma0qkvm7vsks1bpw3owvy6wvsb6nf4mzi5.png)
now from above equation we will have
![250 (4 m/s) + 0 = 250 v + 250 v](https://img.qammunity.org/2020/formulas/physics/middle-school/oqhybsttu52xgqf3yko407fe3vu6o6ki6r.png)
![1000 = 500 v](https://img.qammunity.org/2020/formulas/physics/middle-school/nrc9fqrt4untyxh17euhsnq85l75uo8hn6.png)
![v = 2 m/s](https://img.qammunity.org/2020/formulas/physics/middle-school/qz8np3vfr7l462xpg1r1w4bn5ok6omj49j.png)
so both cars will move together with speed 2 m/s