xet' cac' tap^. gom 6 chu so khac nhau (0,1,a,b,c,d)chon a b c d co 4c8 cach' chon .=> co 70 tap thoa man
goi so can tim` la` abcdef gach ngang o tren dau
a co 5 cach chon
bcdef co 5! cach chon
=> co 5.5!.70=42000 so
ban thong cam may tinh nha to bi lam sao i.k viet co dau dc