pleasee help!! i got 18.02, i don't know what i'm doing wrong

A car traveling north at 10.0 m/s crashes into a car traveling east at 15 m/s at an unexpectedly icy intersection. The cars lock together as they skid on the ice. The two cars have the same mass. What is their combined speed after the collision?

5.0 m/s
15 m/s
9 m/s
25 m/s

Question

pleasee help!! i got 18.02, i don't know what i'm doing wrong

A car traveling north at 10.0 m/s crashes into a car traveling east at 15 m/s at an unexpectedly icy intersection. The cars lock together as they skid on the ice. The two cars have the same mass. What is their combined speed after the collision?

5.0 m/s
       15 m/s
       9 m/s
       25 m/s

anonymous · Answer

9 is the answer you have not taken the combined mass as twice the initial mass