When Louis arrived at his apartment in Europe he found a note pinned to the wall in his study. The note was pinned on a 10 Euro bill. It was from his yankee roommate Jack, who always liked to play games. 

The note said:

“\*This 10 Euro bill is yours if you confirm for me that the dimensions of the empty box on the floor are 15Long X 11Wide X 15High in inches—yes inches! 

The only thing you can use to measure the box is a flat wooden strip sitting next to the box, which is 35 inches long. Using anything in this room only, you can make only ONE mark on this flat wooden strip. There is a small pencil next to the strip. For measurements, you can only contact the strip and the box. Your measurement must be within +/- 0.05 inches.

If you can’t do it leave a 10$ bill for me buddy”.\* 

Imperial units, damn, thought Louis. But he couldn’t let Jack win. So he set about to measure the box.

He made some quick observations. The box was a normal cardboard box (like ones you get from AMAZON.COM). It had no top or bottom. So only 4 sides. Clearly two sides looked like a square and could be 15X15, he concluded. The wooden strip looked like a Ruler with no marking on it whatsoever. Could not bend it either. There was not much else in the room. Some frames on the wall, a study table with an old Coca Cola can, a printer with A4 paper and pens and pencils. 

Then he pulled out a blank paper from the printer and wrote or sketched something. He took out his iPhone 6 plus and checked a couple of things. Then he smiled.

Using something, he made one mark on the wooded strip with the pencil. He took the box and made his first measurement. “YES” he shouted. Then he made some additional checks using the box and the ruler.

He wrote all the procedure down for Jack. He pocketed the 10 Euros and headed to the bar. “I think I deserve a beer”  

[![enter image description here](https://i.stack.imgur.com/rMXbL.jpg)](https://i.stack.imgur.com/rMXbL.jpg)