bonsoir
a = 28/18 x ( 9/12 + 5/12)
= 28/18 x 14/12 = 392/36 = 98/9
b = ( 11/21 + 6/21) x ( 6/10 + 9/10)
= 17/21 x 15/10 = 255/210 = 17/14
c = 1 - ( 8/7 - 15/42)
= 1 - ( 48/42 - 15/42)
= 42/42 - 33/42 = 9/42 = 3/14
d = 5/2 - 3/10 + 24/30
= 75/30 - 9/30 + 24/30 = 90/30 = 3