Question

OABC is a tetrahedron andOA=a,OB=b and OC=c. The points P and Q are such that OA=AP and 2OB=BQ. The point M is the midpoint of PQ. Find (i) AB, (ii) PQ, (iii) CQ, (iv) QM, (v) MB and (vi) OM in terms of a, b and c.