How to calculate tan 5 of 40 feet length

You problem is a lack of understanding of what tan is.

tan is a ratio, it is NOT measured in feet!

tan of 5 degrees stays the same no matter what the other things in the problem are.

You need to explain your question better.   

How to calculate tan 5 of 40 feet length

tan(5^o) = x/40

x = 40 * tan(5^o)

x = 3.5 ft

I'd like to add this to my post:

If the radius of the trigonometric circle: r = 1ft, than tan of 5^o would be  0.087488663526 ft.

But, if r = 40ft, than the tangent is 40X that amount.