А) 1- 23/30 , 1 - 7/10 , 1 - 2/21 , 1 - 1/9 .

Вычислим значения выражений: 

1) 1 - 23/30  = 30/30 - 23/30 = (30 * 1 - 23 * 1)/30 = (30 - 23)/30 = 7/30; 

2) 1 - 7/10  = 10/10 - 7/10 = (10 * 1 - 7 * 1)/10 = (10 - 7)/10 = 3/10; 

3) 1 - 2/21  = 21/21 - 2/21 = (21 * 1 - 2 * 1)/21 = (21 - 2)/21 = 19/21; 

4) 1 - 1/9  = 9/9 - 1/9 = (9 * 1 - 1 * 1)/9 = (9 - 1)/9 = 8/9.