1. **State the problem:** A truck contains 15 boxes, each either red or blue. Red boxes weigh 3 kg each, blue boxes weigh 2 kg each. The total weight is 36 kg. We need to find the difference between the number of red and blue boxes. 2. **Define variables:** Let $r$ = number of red boxes, $b$ = number of blue boxes. 3. **Write equations:** Since there are 15 boxes total: $$r + b = 15$$ The total weight is 36 kg: $$3r + 2b = 36$$ 4. **Solve the system:** From the first equation, express $b$: $$b = 15 - r$$ Substitute into the second equation: $$3r + 2(15 - r) = 36$$ Simplify: $$3r + 30 - 2r = 36$$ $$r + 30 = 36$$ $$r = 6$$ 5. **Find $b$:** $$b = 15 - 6 = 9$$ 6. **Find the difference:** $$|r - b| = |6 - 9| = 3$$ **Answer:** The difference between the number of red and blue boxes is 3. Therefore, the correct choice is D.