5.543 Найдите значение выражения: a) (5/6 + 4/9) - 5/6 * 4/9; б) 2/8 * 8/8 + (3 1/3 - 2 3/5) : 7/15.

a) (15/18 + 8/18) - 20/54 = 23/18 - 10/27 = (69 - 20)/54 = 49/54.
б) 1/4 + (10/3 - 13/5) * 15/7 = 1/4 + ((50-39)/15) * 15/7 = 1/4 + (11/15) * 15/7 = 1/4 + 11/7 = (7 + 44)/28 = 51/28 = 1 23/28.