piacia

N被10除余1，被3除余2，问被30除余几，选项大概有9,10,11,12，2，

N = 10m + 1; possible numbers: 1, 11, 21, 31, 41
N = 3n + 2; possible numbers 2, 5, 11, ... , 20, ... ,

So N = 30p + 11 because for N<=30, only the number 11 satisfies both N = 10m + 1 and N = 3n + 2.