(4/5) of number A is equal to (4/7) of number B. Given that number B is 4.2, what is the number of A?
甲数的(4/5)等于乙数的(4/7)，已知乙数是4.2，甲数是多少．
<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">(4/7) / (4/5)</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">4.2 * (5/7)</gadget>
<output>3</output>

<result>3</result>