A)
ab+c=1 /-c
ab=1-c /:a
b=(1-c) :a
B)
(x+y) /2=c /*2
x+y=2c /-y
x=2c-y
C)
k(l+m)=t
kl+km=t /-kl
km=t-kl /:k
m=(t-kl):k
