I would think 10 but I would have to see the picture
v1 = 6m/s
v2 = 0
∆v = v1 - v2 = 6m\s
s = t * v = 15m
t = s\v1 = 15(m) \ 6(m\s) = 2.5s
a = ∆v\t = 6(m\s) \ 2.5s = 2.4m\s2
a = F\m = 2.4m\s2
F = a * m = 2.4m\s2 * ?kg
I can't tell you this because I don't know the mass of this cyclist
However here coefficient of friction is not given so unable to calculate.
You can calculate it by below formula.
The formula given by

Or

u is coefficient of friction
N is normal reaction.
Here..
box doesn't move means the force if limiting friction is greater than applied force.
Explanation:
Since, the gravity in moon is 1/6th that of earth.
Therefore,
5m ladder if climed on moon will be equal to 1/6th that on earth .i.e 5×1/6=0.83m