A nurse counts 18 beats of a patient's pulse in 15 s…

Step 1. Period. $T = \dfrac{15\ \text{s}}{18} = 0.833\ \text{s}$.

Step 2. Frequency. $f = \dfrac{1}{T} = \dfrac{1}{0.833\ \text{s}} = 1.2\ \text{Hz}$. We can also count directly: $\dfrac{18}{15\ \text{s}} = 1.2$ beats each second.

Step 3. In one minute. $1.2\ \text{Hz} \times 60\ \text{s} = 72$ beats.