Last year, a certain product was sold at the original price at a profit of 25%, but this year it was sold at 90% of the price. As a result, the number of units sold per month increased by 3 times compared to before. It is known that the monthly profit this year has increased by 840 yuan. How much is the monthly sales?
某商品去年按原价出售可以获利25%，今年按定价的90%出售，结果每月售出件数比以前增加了3倍，已知今年每月的利润反而增加了840元，求今年的每月销售为多少元？
<gadget id="calculator">25 / 100</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 + (1/4)</gadget>
<output>5/4 = around 1.25</output>

<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">(5/4) * (9/10)</gadget>
<output>9/8 = around 1.125</output>

<gadget id="calculator">(9/8) - 1</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">(1/8) * 3</gadget>
<output>3/8 = around 0.375</output>

<gadget id="calculator">(3/8) - 0.25</gadget>
<output>0.125</output>

<gadget id="calculator">0.125 / 0.25</gadget>
<output>0.5</output>

<gadget id="calculator">840 / 0.5</gadget>
<output>1_680</output>

<result>1_680</result>