If you walk for $45$ minutes at a [rate](https://artofproblemsolving.com/wiki/index.php/Rate) of $4 \text{ mph}$ and then run for $30$ minutes at a rate of $10\text{ mph}$, how many miles will you have gone at the end of one hour and $15$ minutes?
$\text{(A)}\ 3.5\text{ miles} \qquad \text{(B)}\ 8\text{ miles} \qquad \text{(C)}\ 9\text{ miles} \qquad \text{(D)}\ 25\frac{1}{3}\text{ miles} \qquad \text{(E)}\ 480\text{ miles}$
$45$ minutes is $\frac{3}{4}$ of an hour, so the walking contributes $\frac{3\text{ hr}}{4}\times \frac{4 \text{ miles}}{1\text{ hr}}=3\text{ miles}$.
Similarly, $30$ minutes is $\frac{1}{2}$ of an hour, so the running adds $\frac{1\text{ hr}}{2}\times\frac{10\text{ miles}}{1\text{ hr}}=5\text{ miles}$.
Their total is $3\text{ miles}+5\text{ miles}=8\text{ miles}$, which is $\boxed{\text{B}}$