The average of the four numbers is 15. If each number is increased by 3, what is the average of the four new numbers obtained?
四个数的平均数是15，如果每个数增加3，那么所得的四个新数的平均数为多少．
<gadget id="calculator">15 + 3</gadget>
<output>18</output>

<result>18</result>