2 times the number A is 21 less than 5 times the number A, what is the number A?
甲数的2倍比甲数的5倍少21，甲数是多少？
<gadget id="calculator">5 - 2</gadget>
<output>3</output>

<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<result>7</result>