Volume=legnthn times width times height a cubes legnth=width=height therefor volume=legnth^3 volume is greather than 125 125<legnth^3 cube root both sides 5<legnth th elegnth is anything greather than 5 an example of some numbers are 6,7,30,3240958049238043284023848239048023843,
First draw the box with the diagonal, as you can see it is split into 2 equal right triangles. Now use the pythagorean theorem (a^2+b^2=c^2) to find the length of the diagonal
