To solve this problem, use the work rate method: find how much of the tank each pipe fills/empties per hour, then combine the rates.

Step 1: Find individual rates (fraction of tank per hour)

Pipe 1 fills the tank in 20 hours:

Pipe 2 fills the tank in 30 hours:

Drain empties the tank in 60 hours (negative rate):

Step 2: Find combined net rate

When all three are open, add the rates:

Step 3: Find common denominator and simplify

The LCD of 20, 30, and 60 is 60:

Step 4: Calculate time to fill one tank

If the net rate is \(\frac{1}{15}\) tank per hour, then:

Answer: The tank fills in 15 hours (Option C)

(3x/4) ÷ (2/5) = (3x/4) × (5/2) = 15x/8

Total increase = (1.2 × 1.1 - 1) × 100 = (1.32 - 1) × 100 = 32%

Let stock = 100. 40 units at 10% profit = 44. 60 units at 5% loss = 57. Total SP = 101. Profit = 1%

Total distance = 60×2 + 40×3 = 120 + 120 = 240 km. Total time = 5 hours. Average = 240/5 = 48 km/h

Amount = 8000(1 + 0.12)² = 8000 × 1.2544 = Rs. 10035.20

Total distance = 240 + 160 = 400m. Speed = 400/20 = 20 m/s = 72 km/h

Let original number = x. x × 1.20 = 480. x = 480/1.20 = 400

Net rate = 1/8 - 1/12 = (3-2)/24 = 1/24. Time = 24 hours

Upstream speed = 12 - 3 = 9 km/h. Time = 45/9 = 5 hours