Xiao Ming bought 7 CDs and 42 floppy disks for 252 yuan, 4 yuan for each floppy disk, how much is a CD.
小明买7张光盘和42张软盘用了252元，每张软盘4元，一张光盘多少元．
<gadget id="calculator">42 * 4</gadget>
<output>168</output>

<gadget id="calculator">252 - 168</gadget>
<output>84</output>

<gadget id="calculator">84 / 7</gadget>
<output>12</output>

<result>12</result>