A child starts from rest at the top of a 4.00m high slide. (a) what is her speed

Question

A child starts from rest at the top of a 4.00m high slide. (a) what is her speed at the bottom if the slide isfrictionless? (b) If she reaches the bottom with a speed of 6.00 m/s, what percentage of her total energy at the top is lost as a result of friction?
A. I've got.
PE = KE
mgh = 1/2 mv^2
v= sqrt of 2gh - 8.85 m/s

B. ??? I found some online help, but neither have the right answer.
PE - KE
mgh - 1/2 mv^2
39.2 - 18= not the right answer (54.1% in the book).
I also tried going back to W=Fs except we don't have the mass of the child. So I don't think that's it. Thank you!

Explanation / Answer

second part percentage lost = (mgh - 0.5mv^2 )/ mgh = 54.1%

percentage lost = energy lost due to friction / total energy

= ( mgh - 0.5mv^2) / mgh =( gh - v^2)/gh

on substituting we get 54.1%

first part is correct

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