Add 4 times of number A to the product of 2.1 times 3, the sum obtained is exactly 7.5 times of this number, find number A.
甲数的4倍加上2.1乘3的积，所得的和正好是这个数的7.5倍，求甲数．
<gadget id="calculator">2.1 * 3</gadget>
<output>6.3</output>

<gadget id="calculator">6.3 / 3.5</gadget>
<output>1.8</output>

<result>1.8</result>