1. **Problem statement:** Karl has 25 children at a party, and each child gets 250 mL of juice. We need to find how many litres of juice he will need. 2. **Formula and rules:** To find total juice needed, multiply the number of children by the amount of juice per child. 3. **Calculation:** $$\text{Total juice in mL} = 25 \times 250 = 6250\text{ mL}$$ 4. **Convert mL to litres:** $$1\text{ litre} = 1000\text{ mL}$$ $$\text{Total juice in litres} = \frac{6250}{1000} = 6.25\text{ L}$$ 5. **Answer for part (a):** Karl needs 6.25 litres of juice. 6. **Problem statement for part (b):** Juice comes in jugs of 1.5 L. How many jugs does Karl need to buy? 7. **Formula:** Number of jugs needed = total juice needed divided by jug size, rounded up because partial jugs cannot be bought. 8. **Calculation:** $$\frac{6.25}{1.5} = \frac{\cancel{6.25}}{\cancel{1.5}} = 4.166\ldots$$ 9. Since Karl cannot buy a fraction of a jug, he must buy 5 jugs. 10. **Final answer:** Karl needs to buy 5 jugs of juice.