Sleeping Babies Dr. Richard Ferber, a pediatric sleep expert, has developed a method* to help children, 6 months of age or older, sleep through the night. Often called "Ferberizing," it calls for parents to wait for increasing lengths of time before entering the child's room at night to comfort the crying child. The suggested waiting time depends on how many nights the parents have been using the method and may be found using the equation
$$
w=5 n+5
$$
where $W$ is the waiting time in minutes and $n$ is the number of the night. For example, on the first night, $n=1$, on the second night, $n=2$, and so on.
a) How long should parents wait on the first night?
b) How long should parents wait on the fourth night?
c) On what night should parents wait 30 minutes?
d) On what night should parents wait 40 minutes?