To get the number 8 by using the numbers 2, 4, and 5, and each number only once, you can use the following arithmetic expression:

$(5 + 4) - 2 = 8$

Explanation:

1. Add 5 and 4 to get 9.
2. Subtract 2 from 9 to get 8.

Would you like further explanations or more possible combinations?

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Related Questions:

1. Can you find another way to make 8 with these numbers using different operations?
2. What happens if you are allowed to use exponents or roots?
3. Can you make 8 using a fractional operation with 2, 4, and 5?
4. What if you include multiplication in the operations? Can you still make 8?
5. How does this problem change if you can use one number more than once?

Tip: Always try combining operations in creative ways, like grouping numbers or testing order of operations (PEMDAS).