The average age of a husband and a wife is 30 years when they were married ten years ago but now the average age of the husband, wife and child is 30 years(the child was born during the interval). What is the present age of the child? Choose the correct choice.
A) 2 years  B) 5 years  C) 10 years  D) 4 years  E) 1 years
40 * 2= 
<gadget id="calculator">40 * 2</gadget>
<output>80</output>
80   30 * 3= 
<gadget id="calculator">30 * 3</gadget>
<output>90</output>
90   -----------   10 years   Answer:10 years
<result>C</result>
The average age of a husband and a wife is 30 years when they were married ten years ago but now the average age of the husband, wife and child is 30 years(the child was born during the interval). What is the present age of the child?