Plant rice seedlings in a rice field with an area of 15 hectares, and the planting will be completed in 5 days. At the same speed, how many days will it take to transplant a 21-hectare paddy field?
给一块面积是15公顷的稻田插秧，5天插完．按照同样的速度，给一块21公顷的稻田插秧，需要几天才能够插完？
<gadget id="calculator">15 / 5</gadget>
<output>3</output>

<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<result>7</result>