If simple reaction time takes an average of 0.17 seconds and discrimination reaction time takes an average of 0.26 seconds, then according to donders' method, how long does the mental event of discrimination take?

The mental event of discrimination would be the difference between the discrimination reaction time and the simple reaction time. In other words, mental event discrimination is 0.26 minus 0.17 seconds. The answer is that the mental event of discrimination would take 0.09 seconds.