The sum of 3.9 and 2.58 is:

$3.9 + 2.58 = 6.48$

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Here are 5 related questions:

1. What is the sum of 7.85 and 3.2?
2. How do you subtract 2.58 from 5.9?
3. What is the result of adding 12.345 and 6.789?
4. Can you find the difference between 8.15 and 3.7?
5. How do you add two decimal numbers step by step?

Tip: When adding decimals, it's important to align the decimal points before performing the addition.