Xiao Ming buys an exercise book with a unit price of 0.9 yuan, and pays 1 yuan, how much should he get back.
小明买一本单价0.9元的练习本，付1元钱，应找回多少元．
<gadget id="calculator">1 - 0.9</gadget>
<output>0.1</output>

<result>0.1</result>