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jmc

number theory junior

Problem

A bus comes by Jerry's bus stop every 20 minutes starting at exactly 5:13 a.m. If Jerry shows up at exactly 8:35 a.m., how many minutes will he have to wait for the next bus?
Solution
Since 20 minutes evenly divides 60 minutes (which is an hour), the bus stops every 13 minutes, minutes, and minutes past the hour. So after 8:35 AM, the next time that the bus stops is 8:53 AM, so Jerry must wait minutes.
Final answer
18