(a) If I do 2 units per day, my roommate will do 1 unit perday. Together we do 3 units per day. Since, we take 30 days to finish the complete work, so total work must be 30 × 3 = 90 units. Now working @ 2 units/day, I will take 45 days for 90 units and my roommate will take 90 days.