To build a road, it is planned to build 4.2 kilometers per day, and it will be completed in 15 days. In fact, an extra 2.8 kilometers will be repaired per day. How many days will it actually be completed?
修一条公路，计划每天修4.2千米，15天修完，实际每天多修2.8千米，实际多少天修完？
<gadget id="calculator">4.2 + 2.8</gadget>
<output>7</output>

<gadget id="calculator">4.2 / 7</gadget>
<output>0.6</output>

<gadget id="calculator">15 * 0.6</gadget>
<output>9</output>

<result>9</result>