coal mine wants to transport a batch of coal to a power plant. If it transports 400 tons per day, it will not be finished in 11 days, and there will be a surplus in 12 days; Surplus; if A ton is shipped every day, and it happens to be shipped in A day (A is a natural number), then A=how much.
某煤矿要将一批煤炭运往某发电厂，如果每天运400吨，那么11天运不完，12天时间又有富余；如果每天运420吨，那么10天运不完，11天时间又有富余；如果每天运A吨，恰好A天运完(A为自然数)，则A=多少．
<result>67</result>