For the hygiene and safety of drinking water for students, the school wants to provide each student with a water cup, each cup is 2 yuan, 10% off at Minmin Supermarket, and "buy four get one free" at Zhengda Supermarket. The school wants to buy 3,600 water cups, please be an "advisor" Do the math, which one is cheaper to buy? How much is it cheaper?
为了学生饮水卫生安全，学校想给每个学生配一个水杯，每个水杯2元，百姓超市打九折，正大超市"买四送一"，学校想买3600个水杯，请你当"参谋"算一算，到哪一家购买更便宜？便宜多少元．
<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">2 * 3_600 * (9/10)</gadget>
<output>6_480</output>

<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">3_600 / 5</gadget>
<output>720</output>

<gadget id="calculator">3_600 - 720</gadget>
<output>2_880</output>

<gadget id="calculator">2_880 * 2</gadget>
<output>5_760</output>

<gadget id="calculator">6_480 - 5_760</gadget>
<output>720</output>

<result>720</result>