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

$P$(wheelchair users who had no falls and no features installed) = $\frac{89}{306} = 29.085$%

P(an event) = No of favorable outcomes/ total no of outcomes. Favorable outcomes= wheelchair users who had no falls and no features installed = 89. Total outcomes = total wheel chair users = 306.