Anne, Edward, and Isaac are the three editors of the Perplexus Weekly newspaper. This week, Anne spotted 300 errors, Edward spotted 100 errors, and Isaac spotted 200 errors. Altogether, they spotted 404 errors. Assuming that each error is equally easy to spot, about how many errors did they miss?
This kind of problem appears in Software Engineering -- you have two (or more) groups testing a possibly buggy piece of software, and use the results to estimate how many bugs are still remaining after the tests.