I think haruspex is right that the question is asking how long it will take to stop if the same opposing force from the tugboat continues. The reason is because I think this problem is intended to be equivalent to an object sliding on a frictionless surface. In reality, a barge and a tugboat is not that situation because of the varying resistance of the water. So I think a barge and tugboat is really not a good choice for a problem of this nature.
I also had to look up the equation F = m(v - u)/t because I didn't know what it meant. Maybe I am revealing my age, but I remember V = Vo + at, which would rearrange to F = m(V-Vo)/t. But afifbaha, please use the equation that you listed and are familiar with: F = m(v-u)/t
So as CWatters said, try rearranging the equation to solve for the unknown.
In the first part of the problem, the final velocity is the unknown. In the second part of the problem, the final velocity is given and time is the unknown.
And like CWatters pointed out, 'N' is not a variable; it is a symbol for Newtons - a unit of force, like 'lb' for pounds.
Sorry to be so wordy, but this is my very first post after my introduction.
~ inept new guy