There are two piles of rice in the grain store, with a total weight of 21 tons. It is known that (3/4) of the first pile is equal to the weight of the second pile of rice. Find the weight of the first pile of rice?
粮店有两堆大米，共重21吨，已知第一堆的(3/4)与第二堆大米重量相等，求第一堆大米的重量？
<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 + (3/4)</gadget>
<output>7/4 = around 1.75</output>

<gadget id="calculator">21 / (7/4)</gadget>
<output>12</output>

<result>12</result>