Chapter 3 - Probability - Supplementary Exercises 3.154 - 3.199 - Applying the Concepts - Intermediate - Page 173: 3.178d

$P$(user who has not installed any features, had a fall) = $\frac{20}{109} = 18.3486$%

P(an event) = No of favorable outcome/Total no of outcomes. Favorable outcome = user who has not installed any features, had a fall = 20 users. Total outcomes = Users who have not installed any features = 109.