|f1-f2| = 5 

f1-f2 = 5 

If on increasing f1 ,f2 or decreasing them there is no change in beat frequency then.....

there is only one condition...

f1-f2 = -5  also....

Now add both of then  2f1 = 10  or f1 =5

Hence Tuning fork has frequency equal to 5 Hz