Question 404894
If Jane do all work alone in x minutes, then in 1 minute she can do {{{1/x}}} part of work
It takes Joan 3 times longer than Jane to file reports, so Joan can do all work alone in {{{3x}}} minutes,  then in 1 minute she can do {{{1/(3x)}}} part of work
If they work together 12 minutes, then Jane does {{{12/x}}}part of work, Joan does {{{12/(3x)=4/x}}}part of work. In 12 minutes they did all work, all work is 1.
Equation
{{{12/x+4/x=1}}} Multiply both sides by x
{{{12+4=x}}}
{{{x=16}}}
So Jane needs 16 minutes for all work, Joan needs {{{3*16=48}}} minutes