2,5:4

Do you mean a ratio or a division?

• If it's a ratio (2,5 : 4) with decimal comma: convert to integers: 2.5:4 = 25:40. GCD is 5, so simplified ratio is 5:8.
• If you mean 2.5 ÷ 4: that's 0.625 (which is 5/8).

So the result is 5:8 for the ratio interpretation (or 0.625 / 5/8 for the division interpretation). Which one did you intend?