Pipes A and B can fill a tank in 20 min and 30 min respectively. If both are opened together, how long does it take to fill the tank?

Problem restatement
Combine two constant fill rates to get the time to fill one tank.

Given data

• A's rate = 1/20 tank per min
• B's rate = 1/30 tank per min

Concept/Approach
Add rates for parallel work: total rate = 1/20 + 1/30 = 1/12 tank per min ⇒ time = reciprocal.

Step-by-step calculation
1/20 + 1/30 = (3 + 2)/60 = 5/60 = 1/12Time = 1 / (1/12) = 12 minutes

Common pitfalls
Averaging times instead of adding rates.

Final Answer
12 minutes