What's missing from the problem statement is how elastic/inelastic the collision is. If it's 100% elastic, both cars will experience a 100 MPH change in velocity (to 50 MPH in the opposite direction).

But yes, the textbook is wrong. Unless the brick wall (and only the wall) is 100% inelastic so it "catches" the 100 MPH car, slowing it to 0. Quite unrealistic.