It takes 3 days for a ship to travel from city A to city B, and 4 days to travel from city B to city A. How many days does it take to float a raft without power from city A to city B?
轮船从A城到B城需行3天，而从B城到A城需行4天，从A城放一个无动力的木筏，它漂到B城需多少天．
<gadget id="calculator">3 + 4</gadget>
<output>7</output>

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

<gadget id="calculator">3 + 21</gadget>
<output>24</output>

<result>24</result>