Zhang Gang has savings of 2,000 yuan, and Li Qiang has savings of 400 yuan. If each of them saves 20 yuan a month from now on, how many months later Zhang Gang's savings will be three times that of Li Qiang.
张刚有储蓄2000元，李强有储蓄400元，如果从现在起每月两人各储蓄20元，多少个月后张刚的储蓄数是李强的3倍．
<gadget id="calculator">400 * 3</gadget>
<output>1_200</output>

<gadget id="calculator">2_000 - 1_200</gadget>
<output>800</output>

<gadget id="calculator">20 * 3</gadget>
<output>60</output>

<gadget id="calculator">60 - 20</gadget>
<output>40</output>

<gadget id="calculator">800 / 40</gadget>
<output>20</output>

<result>20</result>