The efficiencies of A, B and C are in the ratio 2 : 5 : 7 working together, they can complete a work in 10 days. In how many days will, A alone be able to complete 30% of that work?

Efficiency ratio 2:5:7 means total efficiency units = 2+5+7 = 14. Working together, they complete the work in 10 days, so total work = 14 × 10 = 140 units. A's efficiency is 2 units/day, so A alone would take 140/2 = 70 days to complete the full work. For 30% of work (0.3 × 140 = 42 units), A would take 42/2 = 21 days. Option D is correct.