Pipe A can fill a tank in 6 hours. Pipe B can fill the same tank in 4 hours. Working together, how long will it take to fill the tank?

Work with rates. Pipe A fills $\frac{1}{6}$ of the tank per hour and Pipe B fills $\frac{1}{4}$ per hour. Together, $\frac{1}{6}+\frac{1}{4}=\frac{2}{12}+\frac{3}{12}=\frac{5}{12}$ of the tank per hour. The time is the reciprocal: $\frac{12}{5}=2.4$ hours, choice D. The answer must be shorter than the faster pipe's time of $4$ hours. Choice B ($10$) adds the individual times, which makes the combined job slower than either pipe alone. Choice C ($5$) averages the times. Choice A ($2$) assumes both pipes take the same time of $4$ hours or a wrong rate.