Answer:
255 Hz
Explanation:
The beat frequency is the difference in frequency between two frequencies that are close.
Let the the unknown frequency be [tex]f[/tex].
Sounded with the 250 Hz tuning fork,
[tex]|250-f|=5[/tex]
[tex]f = 255 \text{ or } 245[/tex]
Sounded with the 270 Hz tuning fork,
[tex]|270-f|=15[/tex]
[tex]f = 255 \text{ or } 285[/tex]
The common value of [tex]f[/tex] in both situations is 255 Hz.