Among all the four-digit numbers, how many numbers have the sum of the first two digits and the last two digits equal to 6?
在所有的四位数中，前两位的数字之和与后两位的数字之和等于6的共有多少个？
<gadget id="calculator">7 * 6</gadget>
<output>42</output>

<result>42</result>