Alex's dog eat's 3 cups of food per day.
Why? Let's explain!!!! We now that she feeds him 15 cups every 5 days and 21 every seven days. But there has to be something we are missing. 15 and 5 21 and 7. Wait 15 divided bye 5? 15/5=3. OK so we have a new number three ok so let's try 21 and 7. 21 divided bye 7? 21/7=3 yes soooooooooo she feeds her dog three cups a day. Fwala Problem solved
