The average time to assemble one lock is $60$ sec$.$ The percent rating of the op$$erator is $75\%$ and the allowance for material delays is $5\%.$ Calculate the standard time.

Group of answer choices

$42\mathrm{.}4$ sec

$45\mathrm{.}0$ sec

$47\mathrm{.}3$ sec

$50\mathrm{.}4$ sec