There are 10 students in a class. The average age of 9 of them is 8 years. By how many years will this average increase if the tenth student is (quite strangely) 28 years old?
9 students have an average age of 8 years so the sum of their ages is 9*8 = 
<gadget id="calculator">9*8</gadget>
<output>72</output>
72 years.
A tenth student who is 28 years old will make the sum 72+28 = 100 years.
The new average age will be 100/10 = 
<gadget id="calculator">100/10</gadget>
<output>10</output>
10 years.
This is 10-8 = 
<gadget id="calculator">10-8</gadget>
<output>2</output>
2 years higher than the previous average.

<result>2</result>