Relative Speed Matrix (Head-Start Relative Speed)
GMAT glossary · Playbook
Definition
Collapsing two movers into one relative speed: add speeds when closing, subtract when chasing the same direction, then Distance = relative speed x time.
Relative Speed Matrix (Head-Start Relative Speed) in plain English
Pull the Relative Speed Matrix (also called Head-Start Relative Speed) when two objects move toward or away from each other. Clues include toward each other, opposite directions, same direction, or catch up. Closing (toward / opposite): add speeds. Same-direction chase: subtract speeds. Then Distance = relative speed x time. Tracking two absolute positions is noisy; one relative speed collapses the frame into a single moving gap. Decide first whether the gap is shrinking or growing, then pick add vs subtract. Micro-example: trains 60 mph and 40 mph head toward each other 200 miles apart. Closing speed 100 mph, time = 200/100 = 2 hours. If they traveled the same direction, relative speed would be 20 mph instead.
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