Answer:
19/20 of a mile.
Step-by-step explanation:

The common denominator is 20 so we times the first one by 2 the second one by 5 and the last one by 4. (Both numerator and denominator.)
You will end up with this 
The distance between a point

on the given plane and the point (0, 2, 4) is

but since

and

share critical points, we can instead consider the problem of optimizing

subject to

.
The Lagrangian is

with partial derivatives (set equal to 0)




Solve for

:


which gives the critical point

We can confirm that this is a minimum by checking the Hessian matrix of

:


is positive definite (we see its determinant and the determinants of its leading principal minors are positive), which indicates that there is a minimum at this critical point.
At this point, we get a distance from (0, 2, 4) of
0.45 (weight of each cupcake) *100 (number of cupcakes) = 45
The box of 100 cupcakes will weight 45 pounds. :)