Bonsoir,
a. 2/3 + 8/15 = 6/5
2/3 = (2 x 10) / (3 x 10) = 20/30
8/15 = (8 x 2) / (15 x 2) = 16/30
6/5 = (6 x 6) / (5 x 6) = 36/30
b. 7/2 x 5/14 = 5/4
7/2 x 5/14 = 35/28 = (7 x 5) / (7 x 4) = 5/4
c. 9/8 - 7/16 = 11/16
9/8 = (9 x 2) / (8 x 2) = 18/16
18/16 - 7/16 = 11/16.