2004 AIME II Problems/Problem 5

Problem

In order to complete a large job, workers were hired, just enough to complete the job on schedule. All the workers stayed on the job while the first quarter of the work was done, so the first quarter of the work was completed on schedule. Then workers were laid off, so the second quarter of the work was completed behind schedule. Then an additional workers were laid off, so the third quarter of the work was completed still further behind schedule. Given that all workers work at the same rate, what is the minimum number of additional workers, beyond the workers still on the job at the end of the third quarter, that must be hired after three-quarters of the work has been completed so that the entire project can be completed on schedule or before?

Solution

A train is traveling at miles per hour and has one hour to reach its destination miles away. After minutes and miles it slows to mph, and thus takes minutes to travel the next miles. Then it slows to mph, so the next quarter takes . The train then has minutes left to travel 250 miles, and doing the arithmetic shows that during this last stretch it must travel more than mph to make it on time. Therefore the company needs to add more workers.

Solution by rocketscience