It could be a positive square root l like √10 ( the number not being a perfect square).
He would have obtained this value from the application of the Pythagoras theorem. For example the length and width of the rectangle might have been 3 and 1 foot respectively, so the diagonal would have length √(3^2 + 1^2) = √10.
He could give an estimate of the length to nearest hundredth using his calculator. This would be 3.16 feet.
