A messenger is sent to a town and advances daily by twenty miles. In how many days will another messenger, sent five days later and advancing daily by thirty miles, overtake him? ANSWER: 15
STEP BY STEP: You will do the following. Take the difference between twenty and thirty, namely ten, as the principal. Next, multiply five by twenty and divide the product by the principal; this will give ten, and so many days did the second messenger walk. The first has been on his way that many days plus five, which is fifteen, when the second messenger met him.