Flour is 4.9 yuan per kilogram, and rice is 3.8 yuan per kilogram. If you buy 10 kilograms of flour and 10 kilograms of rice, and pay 100 yuan, how much should you get back?
面粉每千克4.9元，大米每千克3.8元，买面粉和大米各10千克，付出100元，应找回多少元？
<gadget id="calculator">4.9 * 10</gadget>
<output>49</output>

<gadget id="calculator">3.8 * 10</gadget>
<output>38</output>

<gadget id="calculator">49 + 38</gadget>
<output>87</output>

<gadget id="calculator">100 - 87</gadget>
<output>13</output>

<result>13</result>