Question 901672
This might work easier if viewed as a task completion problem instead of a travel uniform rates problem.


Think of the distance between Mitch and Lynn from the place of each as 1 job.
This way, Mitch does this job in 12 minutes and Lynn does this job in 5 minutes.  If they start this job  at the same time then when do they finish (like doing 1 job working together, when do they meet)?


Rates
Mitch, d/12
Lynn, d/5
Think of d=1, ONE FULL DISTANCE, like one whole job.


RT=1, rate time, 1 job  or more generally, RT=D using D to stand for the single distance or a number of or fraction of the single distance.


TIME is the variable.  
Let t = unknown time for Mitch and Lynn to do the job  of the ONE distance.
{{{highlight((1/12+1/5)*t=1)}}}
Reminder, {{{t}}} is in MINUTES.