CEMC Fermat, 2025

Algebra Difficulty 2.5 Junior Find the answer Canada

A substance doubles its volume every minute. At $\text{9:00 a.m.}$ a small
amount of the substance was placed in a large empty container. At $\text{9:20 a.m.}$ the same day, the container became full. At what time was the
container one-quarter full?

Pick one

Solution

Each minute time moves forward, the volume of the substance
doubles, and thus each minute time moves backward, the volume is
halved.

The container was full at 9:20 a.m., and so it was half full at 9:19
a.m., and one-quarter full at 9:18 a.m.

Want a route through all this instead of an archive? The track puts 2,604 problems in a working order, from Junior Challenge level to the IMO shortlist.

Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.