Consider the question. 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?
 If the answer is '21'; explain the reasoning:
James has 28 marbles / 4 bags = 7 marbles per bag. James has 28 marbles - 7 marbles = 21 marbles remaining.