Given 6 consecutive natural numbers, the sum of the first three numbers is 45, what is the average of the last three numbers?
连续6个自然数，前三个数的和是45，后三个数的平均数是多少．
<gadget id="calculator">45 / 3</gadget>
<output>15</output>

<gadget id="calculator">15 + 3</gadget>
<output>18</output>

<result>18</result>