Номер 522

(a + b) : 7
(15 + 13) : 7 = 4;
(7 + 56) : 7 = 9;
(21 + 35) : 7 = 8;
(5 + 9) : 7 = 2;
(20 + 22) : 7 = 6;
(17 + 18) : 7 = 5;
(25 + 24) : 7 = 7.