42 pieces of a product are sold to 20 customers. Each customer can buy up to three pieces. Buy one piece at the original price. Buy two pieces and get a 10% discount. Buy three pieces and get a 30% discount for final settlement. How many customers are there who bought three pieces?
某商品42件，售给20位顾客，每位顾客最多可以买三件，买一件，按原价，买两件打九折，买三件打七折最后结算，恰好相当于所有商品统一8折出售，那么买三件的顾客有几位？
<gadget id="calculator">42 - 20</gadget>
<output>22</output>

<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">22 * (4/5)</gadget>
<output>88/5 = around 17.6</output>

<gadget id="calculator">42 * (4/5)</gadget>
<output>168/5 = around 33.6</output>

<gadget id="calculator">20 * 1</gadget>
<output>20</output>

<gadget id="calculator">(168/5) - 20</gadget>
<output>68/5 = around 13.6</output>

<gadget id="calculator">(88/5) - (68/5)</gadget>
<output>4</output>

<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(4/5) - (3/10)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">4 / (1/2)</gadget>
<output>8</output>

<result>8</result>