How was the figure of 25% calculated? Why not 20 or 30?

It is difficult to say exactly how the 25% figure was calculated without context, but usually choosing 25% instead of 20% or 30% is related to a balance between accuracy and simplicity.

For example, in optimization or performance evaluation tasks, 25% may be chosen as a convenient benchmark, as it is a neat quarter that is easy to perceive and interpret.

If it concerns statistics or distribution, 25% is the first quartile, which makes mathematical sense.

In programming and algorithms, 25% could be the result of data analysis, where exactly a quarter of the sample or load corresponds to a certain threshold.

If the question is related to specific code or an algorithm, then 25% might have been empirically derived from tests or analysis, where 20% was insufficient, and 30% was too much, making 25% a compromise.

Without additional information, it is difficult to give an exact answer, but usually such figures are chosen either based on mathematical considerations (quartile, quarter) or from practical experience and testing.