goi tu' dien deu` la` SABC, chon dinh? la` S
gja? su? mat cat' khoj' chop' cat' SA,SB,SC lan` luot tai A',B',C'
vi`day' lon' = 2lan` day' nho? day' nho? la` duong` trung bjh` cua? cac' mat
hay AA'=SA',BB'=SB',CC'=SC'
den' day thi don gjan? ruj` ban co' the? tjep' tuc lam` tjep' dc chu' j`. chuc' ban thanh` cong