□, △ and ○ are all tens, □*△=1800, △*○=2100, then □*○=how many.
□、△和○都是整十数，□*△=1800，△*○=2100，那么□*○=多少．
<gadget id="calculator">1_800 / 30</gadget>
<output>60</output>

<gadget id="calculator">2_100 / 30</gadget>
<output>70</output>

<gadget id="calculator">60 * 70</gadget>
<output>4_200</output>

<result>4_200</result>