Gọi ab5 là số cần tìm (có gạch trên đầu)

Theo đề cho ta có: 5ab : ab5 = 3 (dư 21)

=> 5ab = ab5 x 3 + 21

=> 5ab = ab5 x 3 + 3 x 7 = 3 x (ab5 + 7)

=> 500 + ab = 3 x (ab5 + 7)

(bó tay. rối ùi)

classes boards books couches telephones parents students engineers baskets doors lamps benches desks bookcases erasers armchairs tables teachers years windows

/s/: books, parents, students, baskets, lamps, desks

/iz/: classes, couches, bookcases

/z/: boards, telephones, engineers, doors, benches, erasers, armchairs, tables, teachers, years, windows