For a three-digit number, the sum of the numbers in the three digits is 12, and the number in the hundreds place is smaller than the number in the tens place 4. The number in the ones place is twice the number in the tens place. Find the three-digit number.
一个三位数，三个数位上的数字之和是12，百位上的数字比十位上的数字小4．个位上的数字是十位上的数字的两倍．求这个三位数．
<result>840</result>