Je te fais les 3 en espérant que tu comprendras:
A = 2/5 - 4/3 + 7/2
A = 12/30 - 40/30 + 105/30
A = 77/30
B = 2/3 - 5/3 x 3/2
B = 2/3 - 15/6
B = 4/6 - 15/6
B = -11/6
C = [-7/4 + 1/2] / [2/7 + 1/4]
C = [-7/4 + 2/4] / [8/28 + 7/28]
C = [-5/4] / [15/28]
C = -5/4 x 28/15
C = -140/60
C = -7/3