What are some OTHER applications of algebra's related rates problems, beyond the typical examples given as textbook exercises? The textbooks usually emphasize rate-time-distance in which an agent moves at different rates between two situations; or two agents move each at a different rate than the other; and other exercises emphasize two or more agents doing a job, each at a different rate. We can generally create a table to show expressions and variables for the agents, the rates, times, and either distances or job quantities. For the "job" type problems, general examples are people doing a job, or pipes filling or emptying a tank, or a machine/machines performing repetetive tasks. I am interested to know what other such applications are possible which are typically not used in the algebra textbooks? Anyone know of any examples based on your experiences, or more rare textbook examples?