a)
7+6×(X:4-18)=25
6×(X:4 -18)=25-7
6×(X:4-18)=18
X:4-18=18:6
X:4-18=3
X:4=3+18
X:4=21
X=21×4
X=84
b)
286+(a-113):5=142
286+(a-113)=142×5
286+(a-113)=710
a-113=710-286
a-113=424
a=424+113
a=537
c)
[93:(X:3)-16×4]×6=186
93:(X:3)-64=186:6
93:(X:3)-64=31
(X:3)-64=93:31
(X:3)-64=3
X:3=3+64
X:3=67
X=67×3
X=201