Three children slide down a hill.

Three children, Aaron (20kg), Beth (25kg) and Charlie (30kg) take off on sleds at the top of a hill. Who will reach the bottom first? (μ=.15)
a)Aaron
b)Beth
c)Charlie
d)All of them reach at the same time

f=μN

The answer is d. my first guess was A because i thought that since Aaron had the lowest mass his weight force, and therefore his normal force, would be the smallest; giving him the least amount of resistive frictional force. Why is d correct?

but how would I do that if i don't know the $\theta$ of the hill? I drew out my free body diagram but i keep getting stuck when try to resolve the components of the forces.
but how would I do that if i don't know the $\theta$ of the hill? I drew out my free body diagram but i keep getting stuck when try to resolve the components of the forces.