Relative Catch-up Time
GMAT glossary · Playbook
Definition
Catch-up time equals head-start distance divided by the speed difference when a faster object must erase a lead.
Relative Catch-up Time in plain English
Use Relative Catch-up Time when a faster object starts behind and must erase a head start. Clues include head start, how long to catch, or leaves t minutes earlier. Blueprint: Catch-up time = head-start distance / speed difference. Simulating minute-by-minute clocks is slow; the gap closes at a constant relative rate, so one division finishes the story. Convert a time head start into distance first (slower speed x head-start time) if the stem gives minutes rather than miles. Micro-example: a bike at 20 mph gets a 5-mile head start on a car at 30 mph. Gap closes at 10 mph, so catch-up time = 5/10 = 0.5 hours.
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