A car travels at half speed for the first 10 minutes and at full speed for the next 10 minutes. It travels a total of 21 kilometers in 20 minutes. How many kilometers per hour can the car travel at full speed?
一辆汽车前10分钟用半速行驶，后10分钟用全速行驶，这20分钟共行驶了21公里．这辆汽车以全速行驶，每小时可以走多少公里．
<gadget id="calculator">1.4 * 60</gadget>
<output>84</output>

<result>84</result>