Question 706376
This problem has a bit of a trick to it.
You have to open the box to make a net and then use pythagoras theorem to find the hypotenuse of a right angled triangle.
The tricky part is how you make the net! 
I will try to describe how.
In the diagram (http://tinypic.com/r/348t2ft/6)
Open the box so that all the long faces are joined along their longest side and the 2 ends are attached in the appropriate place. (Try drawing a diagram)
(sorry I don't know how to do this any better)
The mouse then has to make a journey that crosses two of the long faces which is the hypotenuse of a right triangle with short sides of 30 feet and (16 + 24=)40 feet.
using pythagoras
{{{30^2 + 40^2= 2500}}}
{{{sqrt(2500)=50}}}
so the mouse can complete the journey in 50 feet.
I hope I have helped.