This is the proportion:

height of short stack of nickels   value of short stack of nickels
———————————————————————————————— = ————————————————————————————————— 
height of tall stack of nickels     value of tall stack of nickels
 
But we must first change the 7 feet height of the tall stack to inches.

× = × = 84in

and we change the 25 nickels to money 

× = × = $1.25

And we let x be the value in money of the tall stack.

Now we substitute in the above proportion:

 1.925    $1.25   
 ————— = —————— 
   84       x

Now we cross multiply:

1.925x = ($1.25)(84)

1.925x = $105

Divide both sides by 1.925

x = 

     x = $54.54545455

That rounds to  $54.55.

Edwin