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Can I say,

(a) Take away three from twenty-eight. (Is this a complete sentence?)

(b) Take away three from twenty-eight is / equalls twenty-five.
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If you want to write it like we say it in math class:

Twenty-eight minus three is twenty-five.
Comments  
Vincent Teo(a) Take away three from twenty-eight. (Is this a complete sentence?)
Yes. It's imperative.
Vincent TeoTaking away three from twenty-eight is / equals/ gives twenty-five.
 BarbaraPA's reply was promoted to an answer.