There are 3ft in 1yd
There are 5280ft in 1mi 

Start with the product of the dimensions, each placed over 1:



Multiply by unit multiplier  twice to cancel away the
yd's and bring in ft's: 



Multiply by unit multiplier  twice to cancel away the
ft's and bring in mi's:



Cancel the yd's



Cancel the ft's



All that's left is



or



which reduces to



or as a decimal



-----------------------------------

There are 640 acres in a square mile.

So we start with



Multiply by unit multiplier  once to cancel away the
mi²'s and bring in acres:



Cancel the mi²'s



All that's left is



or



which reduces to



or as a decimal:



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[Be sure to round off these numbers as your teacher told you.]

Edwin