In a group of 68 students, each student is registered for at least one of three classes – History, Math and English. Twenty-five students are registered for History, twenty-five students are registered for Math, and thirty-nine students are registered for English. If only three students are registered for all three classes, how many students are registered for exactly two classes? Choose the correct choice:
A) 13	B) 10	C) 15	D) 8	E) 7
A U B U 15 = A + B + 15 - AB-B15-A15 + AB15   68 = 25 + 25+ 39 -AB-B15-A15 +3 =&gt; AB+B15+A15 = 24   Exactly Two classes = AB+B15+A15 - 3AB15 = 24 -3*3= 
<gadget id="calculator">24 -3*3</gadget>
<output>15</output>
15   Hence 15
<result>C</result>
In a group of 68 students, each student is registered for at least one of three classes – History, Math and English. Twenty-five students are registered for History, twenty-five students are registered for Math, and thirty-nine students are registered for English. If only three students are registered for all three classes, how many students are registered for exactly two classes?