James has 28 marbles.  He puts them into 4 bags.  He puts the same number in each bag.  He then gives one bag away.  How many marbles does James have left?
James has 28 marbles / 4 bags = 
<gadget id="calculator">28/4</gadget>
<output>7</output>
7 marbles per bag.
James has 28 marbles - 7 marbles = 
<gadget id="calculator">28-7</gadget>
<output>21</output>
21 marbles remaining.

<result>21</result>