Impulse/final velocity question confusion

[student07]

I uploaded my question below; basically I am confused in part b) where they show that:
50 kgm/s = (5kg)v2 - (5kg)(-4m/s) then they say that
30 kgm/s = 5kg v2, but shouldn't it be 70 kgm/s = 5kg v2?

As noted at the beginning of the question North is positive and thus since the cart moves south is negative (-4m/s).
Could this be a typo ?

 

[brainpushups]

If south is negative and that was the initial direction of momentum then the second term is written correctly. Note carefully the signs (subtracting a negative on the RHS) so the second line is correct because the equation reads (without units) 50 = 5v + 20.

 

[student07]

yeah I totally missed the subtracting a negative on the RHS that makes more sense now thank you