Question 874694
You are asking two different questions.  I will help for your first question.


Basic Rule is RT=D for rate, time, distance.


given {{{r<s}}}.  Car B at speed s must close the initial separation of d kilometers, since car B is d kilometers behind car A.


________________speed_________time_________distance
Car A____________r_____________t___________rt-d
Car B____________s_____________t___________st


Think about the description above the data table, and how the information is arranged in the data table; and when you are comfortable, you can continue looking at the arithmetic solution steps.
{{{st-rt=d}}}, or equivalently, {{{st=rt+d}}}.

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When car B reaches and overtakes car A, mostly the focus on the rate of approach, relying just on the two rates, r and s.
{{{st-rt=d}}}
{{{(s-r)t=d}}}
{{{highlight(t=d/(s-r))}}}.