Question

how many inches long is the shadow of an R-foot pole,if an r-inch ruler casts an s-inch shadow at the same time ?

Answer 1

Here is your answer.....

These are two cases given : - 

1.for R foot pole making shadow x foot long (say)

2. for r inches ruler making shadow s inches long
both are occuring at the same time so angle of elevation made by the sun will be equal in both cases.
Let that be α°

So, from case 2, tan α= r/s........ i

now,

 from case 1, tan α= R/x.....ii
=> from i and ii ,
r/s=R/x
hence, x=(Rs)/r feet.

** units in inches will get cancel out in equation (i.)

I hope it helps you ^_^