Each sapling is 20 yuan, buy 3 and get 1 free. Pharaoh bought 3 trees at a time and got another one as a gift, which is equivalent to how much cheaper each tree is.
每棵树苗20元，买3棵送1棵．老王一次买3棵，又获赠一棵，相当于每棵便宜了多少元．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">20 * (3/4)</gadget>
<output>15</output>

<gadget id="calculator">20 - 15</gadget>
<output>5</output>

<result>5</result>