The equation is
the rate of the car times the time it took to
reach the destination set equal to the rate of
the bike times the time it took to reach the
same destination. Because the destination is
the same, d is the same for both. The transitive
property tells us that if the distances are the
same, then the rates and times are the same as
well, so we can set them equal to each other.
That's what all that means. ​