Suppose the coefficient of static friction between a quarter and the back wall of rocket car is .330

Suppose the coefficient of static friction between a quarter and the back wall of a rocket car is 0.330. At what minimum rate would the car have to accelerate so that a quarter placed on the back wall would remain in place? .

[amandamoore]

Wrong answer. Here is the same problem, but friction is 0.213.
Find an expression for the normal force on the quarter using its free-body diagram and the x-component of Newton's second law.

ΣFx = max
n = ma
Because the coin does not fall down the car's back wall,
ay = 0
and the y-component of Newton's second law gives the following.
ΣFy = may
fs − mg = 0
fs = mg
To find the minimum required acceleration, amin, substitute
fs ≤ 𝜇sn
with
n = ma.
fs = mg ≤ 𝜇sn → 𝜇sn ≥ mg
𝜇s(ma) ≥ mg
a ≥
g
𝜇s

amin =
g
𝜇s
=
9.80 m/s2
0.213

amin = 46.0 m/s2