2008 AMC 12B Problems/Problem 20

The following problem is from both the 2008 AMC 12B #20 and 2008 AMC 10B #25, so both problems redirect to this page.

Problem

Your mom walks at the rate of feet per second on a long straight path. Trash pails are located every feet along the path. A garbage truck travels at feet per second in the same direction as your mom and stops for seconds at each pail. As your mom passes a pail, he notices the truck ahead of him just leaving the next pail. How many times will your mom and the truck meet?

Solution

Pick a coordinate system where Michael's starting pail is and the one where the truck starts is . Let and be the coordinates of Michael and the truck after seconds. Let be their (signed) distance after seconds. Meetings occur whenever . We have .

The truck always moves for seconds, then stands still for . During the first seconds of the cycle the truck moves by meters and Michael by , hence during the first seconds of the cycle increases by . During the remaining seconds decreases by .

From this observation it is obvious that after four full cycles, i.e. at , we will have for the first time.

During the fifth cycle, will first grow from to , then fall from to . Hence Michael overtakes the truck while it is standing at the pail.

During the sixth cycle, will first grow from to , then fall from to . Hence the truck starts moving, overtakes Michael on their way to the next pail, and then Michael overtakes the truck while it is standing at the pail.

During the seventh cycle, will first grow from to , then fall from to . Hence the truck meets Michael at the moment when it arrives to the next pail.

Obviously, from this point on will always be negative, meaning that Michael is already too far ahead. Hence we found all meetings.

The movement of Michael and the truck is plotted below: Michael in blue, the truck in red.