A sand glass is made from two conical glass sections, joined together by a thin glass tube. It is turned upside down and sand is allowed to pour from the top section into the bottom section. A sand glass has a carefully measured volume of sand placed inside it, such that it takes an exact amount of time for the sand to flow completely from one section to another. As the sand cannot escape the device it can be used over and over again, to measure a particular length of time accurately.
Two sand glasses have been designed to measure 9 minutes and 13 minutes respectively.
How would you use them to measure 30 minutes?