Given: known fork $f_1$ and beat frequency $n = |f_1 - f_2|$. Unknown $f_2 = f_1 \pm n$. To resolve ambiguity: (1) Load unknown fork with wax (decreases $f_2$). If beats increase: $f_2 < f_1$ originally (wax moves it further from $f_1$). If beats decrease: $f_2 > f_1$ (wax brings it closer to $f_1$). (2) File the unknown fork (increases $f_2$). If beats increase: $f_2 > f_1$. If beats decrease: $f_2 < f_1$. This is one of the most frequently tested beat problems in JEE.