Quandaries
and Queries 

I am a parent of 9th grader and very curios myself how this answer is derived. Thank You for your helpful website but was unable to solve can you please explain how this problem is solved. Thank you so much. Brett 

Hi Brett, Pythagoras Theorem is exactly the tool you need, but you need to use it twice. I drew a sketch of a cube with sides of length 4 and labeled some vertices so that A and B are in the position that you describe.
The line from D to B in the diagram completes the triangle DCB, which has a right angle at C. Thus, by Pythagoras' Theorem
and thus
Now consider the triangle ADB
Triangle ABD is also a right triangle, this time with the right angle at D. Hence, using Pythagoras' Theorem again
and thus
So the length is exactly 4 √3 units, which is approx. 6.93 units. Penny 

