A heart does 1.5 J of work in each heart beat. How many times per minute does it beat if it's power is 2 watts.

The power, the work done and the time is related as,
 $P=\frac{W}{t}$
So, the total work done by the heart in one minute will be,
 $W=P\times t$
Thus, on substituting the values we get,
 $$\begin{gathered} W=\left(2 Watt\right)\times \left(60 s\right) \\ W=120 Joule \end{gathered}$$
Thus, the total work done by the heart in 60 sec is $W=120 J$
Now, the number of heart beats per minute will be calculated as,
 $No. of heart beat per minute=\frac{Total work done}{Work done in each beat}$
On substituting the values we get,
 $$\begin{gathered} No. of heart beat per minute=\frac{120 J}{1.5 J} \\ No. of heart beat per minute=80 times \end{gathered}$$