5/9:2/7,7/18:14/27,3/7:21,15:3/5,3:5,6:12.

Первый пример.

5/9 / 2/7 = 5/9 * 7/2 = 5 * 7/ 9 * 2 = 35/18 = 17 + 1 * 18/18 = 1 17/18;

Второй пример.

7/18 / 14/27 = 7/18 * 14/27 = 7 * 27/18 * 14 = 189/252 = 3 * 63/4 * 63 = 3/4; 

Третий пример.

3/7 / 21 = 3/7 * 1/21 = 3 * 1/7 * 21 = 3/147 = 1 * 3/49 * 3 = 1/49; 

Четвертый пример.

15 / 3/5 = 15/1 * 5/3 = 15 * 5/1 * 3 = 75/3 = 25 * 3/1 * 3 = 25/1 = 25;

Пятый пример.

3 / 5 = 3/1 * 1/5 = 3 * 1/1 * 5 = 3/5;

Шестой пример.

6 / 12 = 6/1 * 1/12 = 6 * 1/1 * 12 = 6/12 = 1 * 6/2 * 6 = 1/2.