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Treat “fill per hour” as rates: A=1/6A=1/6 tank/hour and B=1/4B=1/4 tank/hour. Together, rates add: 1/6+1/4=212+312=5121/6+1/4=\frac{2}{12}+\frac{3}{12}=\frac{5}{12} tank/hour. Time is reciprocal of combined rate: t=15/12=125=2.4t=\frac{1}{5/12}=\frac{12}{5}=2.4 hours. Rate problems test proportional reasoning—combining work contributions correctly and avoiding the trap of averaging times. A good habit is to convert each time to a unit rate, sum them for simultaneous work, then invert. For a quick check, note that the combined time must be less than the faster individual time (4 h), and 2.4 h satisfies this. This structure generalizes to more pipes/workers and even to “drain” scenarios by treating outflow rates as negative. CLI’s quantitative reasoning objectives expect students to model real processes numerically, maintain fraction accuracy, and interpret results for plausibility.