Question 994849
A very common type of travel rates exercise problem.  This will be a general solution for just that reason.

<pre>
r      speed of early, slow train
R      speed of fast, late train
t      travel time for fast train, left later than slow train
h      amount of time passed before fast train departed
d      the catch-up distance
</pre>

<pre>
TRAIN         speed       time     distance
Early Slow      r         t+h       d
Late Fast       R         t         d
</pre>

Question is to solve for d.
The unknown variables are t and d, but you do not need to find a value for t. (Unless you WANT to).
Initial system of equations, {{{system(r(t+h)=d,Rt=d)}}}

{{{t=d/R}}}
-
{{{rt+rh=d}}}
{{{r(d/R)+rh=d}}}
{{{r(d/R)-d=-rh}}}
{{{rd/R-d=-rh}}}
{{{d-rd/R=rh}}}
{{{d(1-r/R)=rh}}}
{{{d=(rh)/(1-r/R)}}}
{{{d=(rh)/(R/R-r/R)}}}
{{{d=(rh)/((R-r)/R)}}}
{{{highlight(highlight_green(d=(rhR)/(R-r)))}}}---The answer in symbolic form.


Your question gives you the values to use:
Substitute these and evaluate d:
{{{system(r=30,R=105,h=5)}}}