Initial Room Cost...... $10  $10  $10  =  $30 (a) 

Refund.................  -1   -1   -1  =   -3 (b) 

                       ====  ===  ===     === 

Subtotal                 $9   $9   $9  =  $27 (c) 

 

Bellboy Stole.............                 -2 (d) 

                                          === 

Actual Cost of the Room...                $25 (e) 

 

To get back to $30, you have to add $27(c) + $3(b) to get to back to $30(a). 

 

It's a mistake to factor in the $2(d) amount at that point in the  

logic.  The $2(d) part doesn't come into play into the equation  

to get back to $30(a). 

 

If you want to figure out how the $2(d) amount comes into the equation,  

you have to go back to the $25(e) amount and add back in $2(d) and $3(b)  

to get back to $30(a).