A mobile phone is sold according to the season. If the current price is reduced by 10%, it can still make a profit of 100 yuan; if the current price is reduced by 20%, it will lose 100 yuan. How much is the purchase price of this mobile phone?
一部手机随季节出售，如果按现价降低10%，仍可盈利100元；如果按现价降低20%，则亏损100元．这部手机进价是多少元？
<gadget id="calculator">100 + 100</gadget>
<output>200</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/5) - (1/10)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">200 / (1/10)</gadget>
<output>2_000</output>

<gadget id="calculator">1 - (1/10)</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">2_000 * (9/10)</gadget>
<output>1_800</output>

<gadget id="calculator">1_800 - 100</gadget>
<output>1_700</output>

<result>1_700</result>