To find out how many times Julie's heart beats per minute, we first determine her heart rate in beats per second and then convert that to beats per minute.
First, we know that her heart beats 12 times in 5 seconds.
To find the number of beats per second, we calculate:
\[ \text{Beats per second} = \frac{12 \text{ beats}}{5 \text{ seconds}} = 2.4 \text{ beats per second} \]
Next, we convert this to beats per minute by multiplying by the number of seconds in a minute (60 seconds):
\[ \text{Beats per minute} = 2.4 \text{ beats/second} \times 60 \text{ seconds/minute} \]
Calculating this gives:
\[ \text{Beats per minute} = 2.4 \times 60 = 144 \text{ beats/minute} \]
Thus, Julie's heart beats 144 times per minute.