Write the number 5 to the left of a three-digit number, and add 400 to the obtained four-digit number. At this time, their sum is 55 times the three-digit number. What is the three-digit number?
把数字5写到一个三位数的左边，再把得到的四位数加上400，这时，他们的和是这个三位数的55倍，这个三位数是多少．
<gadget id="calculator">5_000 + 400</gadget>
<output>5_400</output>

<gadget id="calculator">55 - 1</gadget>
<output>54</output>

<gadget id="calculator">5_400 / 54</gadget>
<output>100</output>

<result>100</result>