phase difference at any given instant between two particles that are $20\mathrm{.}0m$ apart? $($Ans: $112\mathrm{.}5)$

Ist identiff $$:$=\frac{2}{k}=\frac{2}{\frac{}{32}}=64m$

$k=\frac{}{4}*\frac{1}{8}$

why $k$ is equale to $\frac{}{16}$

instead of $64?$