Finding a minimum with solver and 3 variables

I am trying to solve the following question:

Minimize A by changing b and d, adding the constraints of absolute Δmax ≤ 5 mm, and b ≥ 125 mm.

Where, A is the cross-section of a rectangular beam and Δ is the defleciton due to a force.

There are the following relations:
A=b*d,
Δis a funciton of I (Δ(I)), where I = (A*d^2)/12.

I have to use solver to answer this question, but whenever I try it just makes A go to 0, which although is true, is not right. I have tried solving this 2 different ways, Minimizing A, and minimizing I, but neither work. Can anybody help?

Post a workbook.