Hi
I also sat this M1 paper and have argued with some other people as to the answer to this question, and many people have many answers.
Im fairly sure that the question did mention friction and not just a resistance.
There is the 10N pulling both the barges up the slope.
The mass of the two barges creates a force of 9.8N down the slope
( (1.5+2)x9.8x(2/7) )
The value of 2/7 is what was provided, Sin(alpha)=2/7
10N - 9.8N leaves a value of 0.2N resultant force to pull the barges up the slope, however the frictional forces then prevent the barges for moving. A lot of people I talked to simple minused the frictional forces, forgetting that that is what they are and do not cause motion, their answer then showed the barges traveling down the slope.
Unfortunatly out of the people I have spoken to, I have not come across a person with the same answer, nor have I come across a person who has managed to justify their answer.
I hope that helped.