Two cars, travelling at the same speed, are involved in separate crashes. driver A is 100kg and driving an old car with 'fixed' seat belts. he decelerates from 30 m/s to rest in 1 second.
Driver B is also 100 kg, but driving a car with more modern 'inertia-type' seat belts. these seat belts extend slightly before bringing the driver to a stop, so they bring the driver to a stop more slowly. driver B decelerates from 30m/s to rest in 3 seconds.

a) calculate the deceleration of both drivers, show your working.
--> I did: driver A: 30-1/10 = 29.9 m/s 2
driver B: 30-3/10= 29.7 m/s 2

Is this the right answer?

b) calculate the force exerted by the seat belts on both drivers.
c) explain why the 'inertia-type' seat belts are safer to way.
d) how might this 'theory' be used to design crash barriers on motorways or at racing circuits ?

I understood the b) but to do it I need to be sure the a) is correct.
thanks!