Given, beat frequency =8Hz. Beat frequency reduces to 4Hz, if tension reduces. Since, f=
1
2l
√
T
µ
⇒f∝√T where, f= frequency, l= length, T= tension and µ= mass per unit length. fA−fB=8 ⇒fB=fA−8 Here, fA and fB are frequency of string A\B. fA=320Hz ∴fB=320−8=312Hz