If the ends span from the 0 to the 15 meter mark, then the length of the pipe is 15 meters. However, you should report it using the right amount of significant figures. It is mentioned in the problem that millimeter marks are calibrated between meters. Since millimeter is 1/1000 of a meter, you should add three decimal places to the 15 meters. Hence, you should report it as 15.000 meters.
