.
The equation you are asking for is
40*(t+3) = 55*t,
where t is the time counted after the second car started.
The equation says that the distance covered by each car from the starting point to the catching point is the same for both cars.
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For introductory lessons on Travel and Distance problem see the lessons
- Travel and Distance problems
- Travel and Distance problems for two bodies moving in opposite directions
- Travel and Distance problems for two bodies moving in the same direction (catching up)
in this site.