A book has 80 pages in total. I read (1/4) of it for the first time and (2/5) of it for the second time. How many pages are left?
一本书共80页，第一次看了它的(1/4)，第二次看了它的(2/5)，还剩多少页没看？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(1/4) + (2/5)</gadget>
<output>13/20 = around 0.65</output>

<gadget id="calculator">1 - (13/20)</gadget>
<output>7/20 = around 0.35</output>

<gadget id="calculator">80 * (7/20)</gadget>
<output>28</output>

<result>28</result>