Solution
= Solution
Write the count as a sum of $24$ <Bernoulli random variables>, one for each attendance indicator. Each <indicator random variable> has <expected value> $3/4$, so <linearity of expectation> gives
$$
\boxed{\mathbb E[N]=24\cdot\frac34=18\ \text{lectures}.}
$$
Equivalently, $N$ has a <binomial distribution> with parameters $24$ and $3/4$.