digs a canal. On the first day, he dug (1/4) of the total length of the canal, on the second day he dug the remaining length of the canal (5/21), on the third day he dug (1/2 ), and finished digging the remaining 100 meters of canals on the fourth day. So, how long is this canal?
甲挖一条水渠，第一天挖了水渠总长度的(1/4)，第二天挖了剩下水渠长度的(5/21)，第三天挖了未挖水渠长度的(1/2)，第四天挖完剩下的100米水渠．那么，这条水渠长多少米．
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">5 / 21</gadget>
<output>5/21 = around 0.238095</output>

<gadget id="calculator">(3/4) * (5/21)</gadget>
<output>5/28 = around 0.178571</output>

<gadget id="calculator">1 - (1/4) - (5/28)</gadget>
<output>4/7 = around 0.571429</output>

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

<gadget id="calculator">(4/7) * (1/2)</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">100 / (2/7)</gadget>
<output>350</output>

<result>350</result>