Answer
$$51 \mathrm{\ ns} $$
Work Step by Step
$\text { We use } \Delta t=d / v, \text { where } d=12 \mathrm{m} . \text { For an antiproton } $
$$\Delta t=\frac{1}{0.785\left(2.998 \times 10^{8} \mathrm{m} / \mathrm{s}\right)}=5.1 \times 10^{-8} \mathrm{s}=51 \mathrm{\ ns} \text { . } $$
