The ratio of A and B is 1:3. If A subtracts 2, the ratio of A and B is 1:5. What is the original number of A?
甲、乙两数的比是1：3，如果甲减去2后，甲与乙的比是1：5，甲数原来是多少．
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/3) - (1/5)</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">2 / (2/15)</gadget>
<output>15</output>

<gadget id="calculator">15 * (1/3)</gadget>
<output>5</output>

<result>5</result>