Suppose that accidents occur in a large industrial plant at a rate of $\lambda = \frac{1}{10}$ per day. Suppose we begin observing the occurrence of these accidents at the starting of the work on Monday. Let $X$ be the number of days until the first accident occurs. Then the distribution of $X$ is: $$ F(x) = \begin{cases} 1 - \mathrm{e}^{-\frac{x}{10}}, & x > 0 \\ 0, & x \leq 0 \end{cases} $$ Now, the source I am referring to says that: The probability that the first week is accident free is $$ P[X \geq 5] = \mathrm{e}^{-\frac{5}{10}} $$ Well my question is why is it not $P[X \geq 7]$? As we are the finding the probability that the first week is accident free that means there should not be accident in the first week and hence my question.
Also, the probability that the first accident occurs on Wednesday of the second week is given as: $$ P[7 \leq X \leq 8] $$ Why is it not calculated between $9$ and $10$?
Maybe they took in account that not all days are working days. In other words, Saturday and Sunday are not included.