To find Maria's heart rate per minute, you can first determine her heart rate for one minute by dividing the number of beats in three minutes by 3.
Given that Maria has a resting heart rate of 240 beats in three minutes, you can calculate her heart rate per minute as follows:
\[ \text{Heart rate per minute} = \frac{240 \text{ beats}}{3 \text{ minutes}} = 80 \text{ beats per minute} \]
So, Maria's heart rate is 80 beats per minute.