We know that $\upsilon \propto \sqrt{T}$ where $\upsilon =$ frequency, T = tension The decrease in the tension of a string decreases its frequency. So let us assume that original frequency uA of A is more than the frequency $\upsilon_B$ of B. Thus $\upsilon_A - \upsilon_B = \pm 6$ Hz (given) and$\upsilon_A = 324$ Hz ∴ $324 - \upsilon_B = \pm 6$ or $\upsilon_B = 324 \pm 6 = 318$ Hz or 330 Hz. On reducing tension of A, $\Delta \upsilon = 3$ Hz If $\upsilon_B = 330$ Hz and on decreasing tension in A, $\upsilon_A$ will be reduced i.e. no. of beats will increase, but this is not so becauseno. of beats becomes 3. ∴ $\upsilon_B$ must be 318 Hz because on reducing the tension in string A, its frequency may be reduced to 321 Hz, thus giving 3 beats with $\upsilon_B = 318$ Hz.