To calculate the unit price for a bottle of water, you need to divide the total cost by the total number of bottles.

Step 1: Find the total number of bottles.

Since Devin buys 5 cases of water, and each case contains 24 bottles: $5 \times 24 = 120 \text{ bottles}$

Step 2: Divide the total cost by the total number of bottles.

Devin paid $20.40 for the 120 bottles: $\frac{20.40}{120} = 0.17$

So, the unit price for each bottle of water is $0.17.

Let me know if you'd like further clarification or have additional questions!

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Here are five related questions you might find interesting:

1. What would the total cost be if Devin bought 10 cases instead of 5?
2. How would the unit price change if the cost were $25.50 instead of $20.40?
3. If Devin bought 120 bottles at a price of $0.18 per bottle, how much would he pay in total?
4. How can you calculate the price per bottle if there is a discount on the case price?
5. What other ways can you use the price per bottle to compare similar products?

Tip: When calculating unit prices, always ensure you're dividing the total cost by the total quantity to find the cost per unit.