**Answer:**

378 m

**Explanation:**

First of all, we have to calculate Sam's acceleration. By using Newton's second law, we can write that the resultant of the forces acting on Sam is equal to the product between his mass and his acceleration (a):

where:

F = 170 N is the thrust, pushing Sam forward

is the force of friction, acting backward against Sam, with

being the coefficient of friction

m = 71 kg is Sam's mass

g = 9.8 m/s^2 is the gravitational acceleration

Solving the equation for a, we find the acceleration:

So, for the first 15 seconds, until he runs out of fuel, he accelerates with this acceleration. Therefore, the distance covered in this part of the trip is

And the speed reached by Sam after these 15 seconds is

Then, the skis run out of fuel; so now there is no more thrust force, and Newton's second law simply becomes

So, the new acceleration is

So the distance covered by Sam in this second part of the trip until he stops is given by

where

v=0 is the final speed

u = 21 m/s is Sam's initial speed

a = -1.0 m/s^2 is the acceleration

Solving for S2, we find

So, the total distance travelled by Sam is