The starting price of a taxi in a certain place is 5 yuan (within 3 kilometers), and after 3 kilometers, the average charge is 1.6 yuan per kilometer. Xiaoming’s family of three took a taxi to grandma’s house and paid a total of 21 yuan. Ask Xiaoming’s home. How many kilometers is the grandma's house in total?
某地出租车起步价为5元(3千米以内)，超过3千米后，平均每千米收1.6元，小明一家三口乘出租车到外婆家共付车费21元，问小明家到外婆家一共有多少千米？
<gadget id="calculator">21 - 5</gadget>
<output>16</output>

<gadget id="calculator">16 / 1.6</gadget>
<output>10</output>

<gadget id="calculator">10 + 3</gadget>
<output>13</output>

<result>13</result>