Suppose a 0.04-kg car traveling at 2.00 m/s can barely break an egg. What is the minimum sped a 0.08-kg car would need to have in order to break the egg?

You need more information--with what you wrote, the 0.08 kg car travelling at 1.00 m/s would have the same effect since both cars would have equal momentum, so it would need to be travelling >1.00 m/s to break the egg, assuming what you wrote is the complete problem