Hello,
m is the mother 's age
a is your age
My mother is 12 years more than twice my age. ==>m=2a+12 (1)
after 8 years she will be 20 years less than three times my age:
in 8 years: mother 's age=m+8
your new age: a+8
m+8=3*(a+8)-20 (2)
m=2a+12 (1)
(2a+12)+8=3*(a+8)-20 (for (2))
==>2a+20=3a+24-20
==>3a+4=2a+20
==>3a-2a=20-4
==>a=16 (your age )
m=2a+12=2*16+12=32+12=44
Proof: m=2a+12==>44=2*16+12
m+8=3(a+8)-20==>44+8=3(16+8)-20 ==>52=72-20