Yes, at degree level I think I'd have given that zero as well.
The tips increase the effective aspect ratio through modification of the Oswald span efficiency factor (e) in the term Cd = Cdo+(1/e.pi.AR)Cl^2.
This effective increase in AR reduces the induced drag term. (NOT the profile drag term, very occasionally called "pressure drag").
Physically, it reduces the energy loss through creation of tip vortices, as admirably demonstrated by some of the pretty pictures above.
So drag is reduced, Cl/Cd is increased, wake vortices reduced thus reducing wake separation requirements, fuel burn is reduced, particularly in low speed flight where the Cl^2 term predominates, although there will still be some effect at high speed (where the Cdo term predominates).
At anything above foundation level, I'd have thought that your lecturers would be looking for an explanation along the lines I've given above, with a sketch diagram of the tip vortices and how they reduce in magnitude.
G