1. We are given three workers A, B, and C with individual times to complete a tank: A takes 12 days, B takes 15 days, and C takes 20 days. 2. First, find each worker's rate per day: - A's rate is $\frac{1}{12}$ tank/day. - B's rate is $\frac{1}{15}$ tank/day. - C's rate is $\frac{1}{20}$ tank/day. 3. To find the combined rate when they work together, sum their rates: $$ \frac{1}{12} + \frac{1}{15} + \frac{1}{20} $$ 4. Find the common denominator which is 60 and rewrite: $$ \frac{5}{60} + \frac{4}{60} + \frac{3}{60} = \frac{12}{60} $$ 5. Simplify the combined rate: $$ \frac{12}{60} = \frac{1}{5} $$ 6. This means together they complete $\frac{1}{5}$ of the work per day, so the total time to finish the tank together is: $$ \text{Time} = \frac{1}{\text{combined rate}} = \frac{1}{\frac{1}{5}} = 5 \text{ days} $$ **Final answer:** It will take 5 days for A, B, and C to finish the work together.