In the formula (□+50)/20*4=80, how much should be filled in □.
在算式(□+50)/20*4=80中，□处应填多少．
<gadget id="calculator">80 / 4</gadget>
<output>20</output>

<gadget id="calculator">20 * 20</gadget>
<output>400</output>

<gadget id="calculator">400 - 50</gadget>
<output>350</output>

<result>350</result>