How do you solve this car ramp cardboard physics problem??

Question

I would like to know how to solve this physics problem because I just wrote an exam and I won't be getting it back.

A 0.5 kg hot wheels car is pushed down a hot wheels track of 4 m high. At the bottom, cardboard is stacked to absorb the impact of the car. It takes 65 N to crush one piece of cardboard (7mm thickness for the cardboard). How many pieces of cardboard are needed to completely absorb the impact without the last piece being crushed.

I hope this is self explanatory, it reads pretty confusing, sorry about that.

Answer 1

P.E =m.g.h

      =0.5 x 9.8 x 4 =19.6

work done =force x distance

               =65 x 0.007 x n

so n =19.6/0.455

Answer 2

Since there is no friction mentioned, it will be assumed that all the energy from the hot wheels car will be absorbed the the cardboards.

The total energy of the hot wheels car = Potential energy = mgh

where

m = mass of the car = 0.5 k.

g = acceleration due to gravity = 9.8 m/sec^2 (constant)

h = initial vertical position of the car = 4 m.

Substituting values,

Potential energy = PE = (0.5)(9.8)(4)

PE = 19.6 joules

The total energy absorbed by the cardboards will be = F(T)(N)

where

F =  force required to crush one piece of cardboard = 65 N (given)

T = thickness of the cardboard = 7 mm = 0.007 m (given)

N = number of cardboards required to absorb the impact

Substituting values,

(65)(0.007)(N) = 19.6

Solving for "N",

N = 43 pieces