Question 947616
THE FIFTH GRADER ANSWER:
Ferdbert is obviously {{{2/5}}} of the tunnel's length from the end the train is coming from,
but he is fast enough to get to that end of the tunnel just as the train gets there.
How long will it take Ferdbert to run that distance? We don't know, and we do not know how long that tunnel is either.
Ferdbert's other option is to run {{{3/5}}} of the tunnel's length in the other direction,
and he will run at the same speed, and will get to that far end of the tunnel just as the train gets there.
How long will it take Ferdbert to run that distance? We don't know either.
What we know is that the difference between
the time it takes Ferdbert to run {{{2/5}}} of the tunnel's length, and
the time it takes Ferdbert to run {{{3/5}}} of the tunnel's length,
is the time that it would take Ferdbert to run {{{3/5-2/5=1/5}}} of the tunnel's length.
That time is also the time that it takes a {{{60mph}}} train to traverse the entire {{{5/5}}} of the tunnel's length.
So, the train is traveling {{{5}}} times as fast as Ferdbert can run,
and Ferdbert can run {{{1/5}}} as fast as the train {{{60mph}}}, or
{{{60mph/5=highlight(12mph)}}} .