There is an error in the question. If A and B finish the work in 60 days. B cannot finish in 36 days after A works for 12 days.... there is some number wrongly given.
Let A do 1/X of making the wall in a day. So A can make wall alone in X days.
Let B do 1/Y of making the wall in a day. So B can make wall alone in Y days.
A+B can do 1/X + 1/Y of wall in a day. Ie., (X+Y)/XY amount in a day
So they take, XY / (X+Y) number of days.
X Y / (X+Y) = 60 => X Y = 60 (X + Y) = 60 X + 60 Y ---- equation 1
A worked for 12 days. So work completed = 12 * 1 / X = 12/X
Remaining work = 1 - 12/X = (X-12)/X
B completes it in 36 days => (X-12) / X = 36 * 1 / Y
(X - 12) Y = 26 X
XY - 12 Y = 26 X
Substitute value of XY from equation 1,
60 X + 60 Y - 12 Y = 26 X
34 X + 48 Y = 0 => X is coming negative...
there is a problem in the question.