Beats

Two vibrating strings ' $A$ ' and ' $B$ ' produce beats of frequency $8 \mathrm{Hz}$. The beat frequency is found to reduce to $4 \mathrm{Hz}$ if the tension in the string $\text{'}A\text{'}$ is slightly reduced. If the original frequency of $A$ is $320 \mathrm{Hz}$, then the frequency of $B$ ' is