The currency of a certain country is very special. There are 4 yuan and 7 yuan coins. Someone has enough 7 yuan coins. There is no need to give change, so he must prepare at least 4 yuan coins.
某国的货币非常特别，有4元和7元硬币，某人有足够多的7元硬币，他要分别到不同的商店购买80元，90元，100元，120元，180元的商品，希望都无须找零钱，那么他最少要准备4元硬币多少个．
<gadget id="calculator">7 * 8</gadget>
<output>56</output>

<gadget id="calculator">80 - 56</gadget>
<output>24</output>

<gadget id="calculator">24 / 4</gadget>
<output>6</output>

<gadget id="calculator">7 * 10</gadget>
<output>70</output>

<gadget id="calculator">90 - 70</gadget>
<output>20</output>

<gadget id="calculator">20 / 4</gadget>
<output>5</output>

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

<gadget id="calculator">100 - 84</gadget>
<output>16</output>

<gadget id="calculator">16 / 4</gadget>
<output>4</output>

<gadget id="calculator">7 * 16</gadget>
<output>112</output>

<gadget id="calculator">120 - 112</gadget>
<output>8</output>

<gadget id="calculator">8 / 4</gadget>
<output>2</output>

<gadget id="calculator">7 * 24</gadget>
<output>168</output>

<gadget id="calculator">180 - 168</gadget>
<output>12</output>

<gadget id="calculator">12 / 4</gadget>
<output>3</output>

<gadget id="calculator">6 + 5 + 4 + 2 + 3</gadget>
<output>20</output>

<result>20</result>