Bonjour je n'ai pas comprit cet exercice merci de m'aider s'il vous plait

bonjour,

a = 1/5 x -4/3 + 7/2
a = -4/15+7/2
a = -8/30+105/30
a= 97/30

b = 4/5 : -3/7 -7/10
b = 4/5 x -7/3 -7/10
b = -28/15 -7/10
b = -56/30 -21/30
b = -77/30

c = 13 /7 -8/7 : -4/5
c = 13/7 -8/7 x -5/4
c = 13/7 + 10/7
c = 23/7

d = 7/3+3/2 x -10/21
d = 7/3 -30/42
d = 7/3 - 10/14
d = 7/3 -5/7
d = 49/21 -15/21
d = 34/21

e = 5/8 -3/4 : -9/16
e = 5/8 -3/4 x -16/9
e = 5/8 +  4 /3
e = 15/24 +32/24
d = 47/24

f = 6/5  - (-1/9) x(-3/4)
f = 6/5 - (+1/12)
f = 6/5 +1/12
f =  72/60 +5/60
f = 77/60