The incubation period of a chicken is 21 days, and the incubation period of a duck is longer (1/3) than that of a chicken. How many days is the incubation period of a duck?
鸡的孵化期是21天，鸭的孵化期比鸡长(1/3)，鸭的孵化期是多少天？
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 + (1/3)</gadget>
<output>4/3 = around 1.333333</output>

<gadget id="calculator">21 * (4/3)</gadget>
<output>28</output>

<result>28</result>