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

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

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