The regular price of a jacket is $ R $ dollars. This price is discounted by $ a $ "percent off" of the regular price. If sales tax, which is $ b $ percent of the selling price, is added on, then the total amount paid for the jacket is given by the equation:

$$ T = \left[ \left( R \right) \left( 1 - \frac { a }{ 100 } \right) \left( 1 + \frac { b }{ 100 } \right) \right] $$

Which is a true statement about the variable $ a $? (No Calculator)