а) (5/6 - 2/3) : 8/9
First, let's find a common denominator for 6 and 3, which is 6. Then we have:
(5/6 - 4/6) : 8/9
1/6 : 8/9
Now, let's divide 1/6 by 8/9:
1/6 ÷ 8/9 = 1/6 * 9/8 = 9/48 = 3/16

б) (1 - 1/2) : (1/2 - 1/3)
(2/2 - 1/2) : (3/6 - 2/6)
(1/2) : (1/6)
(1/2) * (6/1) = 6/2 = 3