Bonjour !
A = 17/24 - (7/8-5/6)
Dénominateur commun : 24
A = 17/24-(7x3/8x3-5x4/6x4)
A = 17/24 - (21/24-20/24)
A = 17/24 - 1/24 = 16/24
A = 16/24 => 16:8 /24:8 = 2/3
B = (17/30-1/5) - (1/6+1/15)
Dénominateur :commun 30
B= (17/30-6/30)-(5/30+2/30)
B = 11/30 - 7/30 = 4/30
B = 2/15