supermarket is running a promotion. Buy 3 of product A and get 1 free, and buy 4 of product B and get 1 free. The unit price of product A is twice that of product B. The money that mother brings is just enough to buy 8 items of product A. If mother still uses the money to buy product B, how many items can she just buy?
某超市搞促销，甲种商品买3送1，乙种商品买4送1，甲种商品的单价是乙种商品的2倍．妈妈所带的钱刚好能买甲种商品8件，如果妈妈仍用这些钱去买乙种商品，刚好能买几件？
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">8 / 4</gadget>
<output>2</output>

<gadget id="calculator">2 * 3</gadget>
<output>6</output>

<gadget id="calculator">2 / 4</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">6 * (1/2) * 5</gadget>
<output>15</output>

<result>15</result>