Two vibrating vibrating  tuning forks  produce progressive waves given by ${\mathrm{y}}_1=4\sin 500\mathrm{\pi t}$ and ${\mathrm{y}}_2=2\sin 506\mathrm{\pi t}$ The number of beats produced per minute is 

For a plane progressive wave $\mathrm{y}=\mathrm{asin}2\mathrm{\pi nt}$ Comparing the given equations with this equation, we get 
$$\begin{gathered} 500=2{\mathrm{n}}_1\text{ or }{\mathrm{n}}_1=250 \\ 506=2{\mathrm{n}}_2\text{ or }{\mathrm{n}}_2=253 \\ \text{ number of beats }={\mathrm{n}}_2-{\mathrm{n}}_1 \\ = 3 \mathrm{beats}/\sec \mathrm{So}, \mathrm{the} \mathrm{number} \mathrm{of} \mathrm{beats} \mathrm{per} \mathrm{minut} \\ = 3\mathrm{x} 60 = 180 \mathrm{beats}/\mathrm{minute} \end{gathered}$$