Let Length of each Plan A workout be
and Length of each Plan B workout be $b$
on Wednesday,
$5a+3b=7$
$2a+12b=19$
multiply equation one by $4$ and subtract equation two from it
to get,
$18a=9$ or $a=\frac12$
substitute $a$ in eq 2. to get $b$, $12b=18\implies b=\frac32$