Rate of A = 1/12, Rate of B = 1/15. Combined rate = 1/12 + 1/15 = 5/60 + 4/60 = 9/60 = 3/20. Time = 20/3 = 6.67 hours (approximately 6.86).
Original rate: 1/4 job in 5 days = 1/20 per day. New rate: 1.25 × 1/20 = 1/16 per day. Remaining 3/4 job at 1/16 rate = (3/4)/(1/16) = 12 days.
Average of first n natural numbers = (n+1)/2 = 10.5. Therefore, n+1 = 21, so n = 20. Wait, if n=20, average = 21/2 = 10.5. But option shows B=21. Recalculating: (n+1)/2 = 10.5 gives n = 20. Let me verify with n=21: (21+1)/2 = 11. For average 10.5: n=20.
Upstream speed = 50/5 = 10 km/h. Downstream speed = 80/4 = 20 km/h. Boat speed in still water = (10 + 20)/2 = 15 km/h.
Total weight = 3 × 70 = 210 kg. C's weight = 210 - 75 - 68 = 67 kg. Wait, option shows 69. Recalculating: 210 - 75 - 68 = 67. But if answer is A (69), total would be 207. Let me verify: 210 - 75 - 68 = 67 kg.
Sum needed for average 85 in 4 subjects = 4 × 85 = 340. Sum of 3 scores = 75 + 82 + 88 = 245. Fourth subject score = 340 - 245 = 95.
Total distance = 120 + 180 = 300 km. Total time = 2 + 3 = 5 hours. Average speed = 300/5 = 60 km/h.
Sum of 6 numbers = 6 × 18 = 108. Sum of 3 numbers = 3 × 24 = 72. Total sum = 180. Total numbers = 9. New average = 180/9 = 20.
Total cost = (10 × 50) + (15 × 40) = 500 + 600 = 1100. Total items = 25. Average = 1100/25 = ₹44.
Sum of 5 friends = 5 × 24 = 120. Sum of 4 friends = 4 × 22 = 88. Age of friend who left = 120 - 88 = 32 years.