One pipe fills a tank in 4 hours, and another fills it in 12 hours. How long will both pipes take to fill the tank together?

The correct answer is A. 3 hours

Rates: 1/4 + 1/12 = 3/12 + 1/12 = 4/12 = 1/3 of the tank per hour, so both pipes take 3 hours. Adding the times (16 hours) or averaging them (8 hours) are common errors.