A safety researcher has found that fourth to sixth graders engaging in unorganized activities on school grounds experienced injuries at the rate of $2.4$ injuries per 100 thousand student-days. On the average, how many student-days elapse between injuries to fourth to sixth graders in unorganized activities at school, and what is the probability that the next such injury will occur before 45 thousand student-days have passed?

Of the $1.0$ million persons of voting age in Maine, $72 \%$ voted in the 2008 presidential election. For a randomly selected group of 30 Maine residents who were of voting age at that time, what is the probability that at least 20 of these persons voted in the presidential election?

For itemized tax returns in the $\$ 60,000-\$ 75,000$ income group for the most recent year reported, the average charitable contribution was $\$ 1935$. Assume a normal distribution and a standard deviation of $\$ 400$.

d. What level of contributions was exceeded by $40 \%$ of the itemized returns in this income group?

For itemized tax returns in the $\$ 60,000-\$ 75,000$ income group for the most recent year reported, the average charitable contribution was $\$ 1935$. Assume a normal distribution and a standard deviation of $\$ 400$.

c. What level of contributions was exceeded by only $20 \%$ of the itemized returns in this income group?

For itemized tax returns in the $\$ 60,000-\$ 75,000$ income group for the most recent year reported, the average charitable contribution was $\$ 1935$. Assume a normal distribution and a standard deviation of $\$ 400$.

For itemized tax returns in the $\$ 60,000-\$ 75,000$ income group for the most recent year reported, the average charitable contribution was $\$ 1935$. Assume a normal distribution and a standard deviation of $\$ 400$.

a. For a randomly selected household from this income group, what is the probability that the household made charitable contributions of at least $\$ 1600$ ?