n is 1 n in 1,5 n mod 10 in 1,2 and n mod 100 not in 11,12 n mod 10 is 1 and n mod 100 is not 11 n mod 10 is 2 and n mod 100 is not 12 n mod 10 is 3 and n mod 100 is not 13 n in 11,8,80,800 n in 1,3 n is 2 n is 4 n is 1 n in 2,3 n is 4 n is 1 n in 2,3 n is 4 n is 6 n in 1,5,7,8,9,10 n in 2,3 n is 4 n is 6 n is 1 n in 2..9 n in 10..19,100..199,1000..1999