Marky owns a computer accessory shop. For this month, half of their sales are wireless mice, one-fourth are optical mice, and the rest are trackball mice. If Marky's shop was able to sell a total of 80 mice, how many of them are trackball mice?
There were 80/2 = 
<gadget id="calculator">80/2</gadget>
<output>40</output>
40 wireless mice sold.
There were 80/4 = 
<gadget id="calculator">80/4</gadget>
<output>20</output>
20 optical mice sold.
So, there were a total of 40 + 20 = 
<gadget id="calculator">40+20</gadget>
<output>60</output>
60 wireless and optical mice sold.
Therefore, 80 - 60 = 
<gadget id="calculator">80-60</gadget>
<output>20</output>
20 trackball mice were sold this month.

<result>20</result>