To find the unit price per bottle of water, we can use the following steps:

1. Determine the total number of bottles:
   Devin buys 5 cases of water, with 24 bottles per case. So, the total number of bottles is: $5 \, \text{cases} \times 24 \, \text{bottles per case} = 120 \, \text{bottles}$

2. Find the unit price per bottle:
   Devin pays a total of $20.40 for 120 bottles. To find the unit price, divide the total cost by the total number of bottles: $\text{Unit price} = \frac{\$20.40}{120 \, \text{bottles}} = 0.17 \, \text{per bottle}$

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

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Would you like further details or clarifications?
Here are a few related questions:

1. How much would Devin pay for 10 cases of water at the same price per bottle?
2. If the price of a case was $4.50, how many bottles would that case contain?
3. How much does Devin pay per case of water?
4. If Devin buys 200 bottles, how much would it cost him?
5. What is the total price for 50 bottles?

Tip: Always break down the problem into smaller steps (like calculating the total number of bottles) to make sure you understand the math involved.