Correct option is B
Total work = 100% Work done by (P+Q) = 70% Work done by (Q + R) = 50% Work done by (P+Q) and (Q + R) = 70% +50% = 120% Work done by (P+Q+R) = 100% now, (P+Q) + (Q+R) - (P+Q+R) = 120-100 = 20% Q = 20% Work done by P = 70-20 = 50% Work done by R = 50-20 = 30% P works 50% of the total work in same time so P will finish the work first.