There are 30 crickets and 30 grasshoppers in a cage. Every time the red-haired magician transforms, 4 of the katydids will turn into 1 cricket; every time the green-haired magician transforms 5 of the crickets into 2 katydids. After the two magicians changed 18 times in total, there were only grasshoppers and no crickets in the cage. How many grasshoppers are there at this time?
笼子里有30只蛐蛐和30只蝈蝈．红毛魔术师每变一次，会把其中的4只蝈蝈变成1只蛐蛐；绿毛魔术师每变一次会把其中的5只蛐蛐变成2只蝈蝈．两个魔术师一共变了18次后，笼子里只有蝈蝈没有蛐蛐了．这时蝈蝈有多少只．
<gadget id="calculator">18 * 3</gadget>
<output>54</output>

<gadget id="calculator">60 - 54</gadget>
<output>6</output>

<result>6</result>