I've just read this thread. Although Bookworm is correct when he says adding vectors 020/30 to 160/30 does not produce xxx/30, vector addition does not apply to the question asked. Interpolation is what's required, and interpolating between 020/30 and 160/30 will always give xxx/30.
Pianorak: it would appear you have no problem with interpolating the speed, just direction. For the direction, draw a diagram and estimate the direction of the answer. Then do the maths. All the methods described above should work, but if they give an answer too far away from your estimate, then you know you've done something wrong. For interest, this technique is called a 'gross error check', and if you haven't used it already, it should come in handy in your navigaiton training - estimate the expected heading, then compare the estimation with the computed answer - if the results are different, find out why.
All the Best with your exams,
A