Seven and thirty twp hundreths plus two and one tenth times two and seven tenths minus 18a

 Write the expression as:

" 70  +
                         2 * 2 − 18a " ; 
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or;  write as:
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           " 70.32  + (2.1) * (2.7)  − 18a " ; 
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To simplify:
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  Using "PEDMAS" (the "order of operations") ; 

the "multiplication" comes first; 

So:  →  "(2.1) * (2.7) =  5.67 " .

And rewrite:
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 " 70.32  + 5.67 − 18a " .

Now:  " 70.32 + 5.67 = 75.99 " ; 

So, we can the final simplified expression as:
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   " 75.99  −  18a " ;  

or; write as:  " 75  −  18a " .
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The first digit is in the middle school 100s place