24.45 rounded to the nearest whole number is 24.
We had to round down because 24.45 is closer to 24 than to 25.
If you were to subtract 24.45 from 25, you would get 0.55. Subtracting 24 from 24.45 would give you -0.45 (or 0.45). 0.45 is smaller than 0.55