At least a third of Fred's marbles are dark blue. All of the rest are red, except for 4 that are green. If he has 63 marbles, how many of them are red?
Fred has 63/3 = 
<gadget id="calculator">63/3</gadget>
<output>21</output>
21 dark blue marbles.
Fred has 63-21 = 
<gadget id="calculator">63-21</gadget>
<output>42</output>
42 red and green marbles.
Fred has 42-4 = 
<gadget id="calculator">42-4</gadget>
<output>38</output>
38 red marbles.

<result>38</result>