Write the number 8 in front of a three-digit number to get a four-digit number. This four-digit number is exactly 21 times the original three-digit number. What is the original three-digit number?
把数字8写在一个三位数的前面，得到一个四位数，这个四位数恰好是原三位数的21倍，原三位数是多少．
<gadget id="calculator">21 - 1</gadget>
<output>20</output>

<gadget id="calculator">8_000 / 20</gadget>
<output>400</output>

<result>400</result>