Explanation:
Given that,
2 strings both vibrate at exactly 220 Hz. The frequency of sound wave depends on the tension in the strings.
The tension in one of them is then decreased sightly, then [tex]f_2[/tex] will decrese.
Beat frequency, [tex]f=f_1-f_2[/tex]
[tex]3=220-f_2[/tex]
[tex]f_2=217\ Hz[/tex]
So, the new frequency of the string is 217 Hz. Hence, this is the required solution.