Car A is on X axis and Car B is on Y. They hit at right angles (mid point of A) and both travel on X axis for 10m before stopping (still together creating a T shape). Assuming they both weigh the same, what speed was car B travelling at the point of impact?

Assuming there are no constraits that keep the two cars confined to moving along the x-axis only after the collision, then the speed of B was zero.