Let B = unknown weight X
then A = 3 times that
So 3x + 1x = 80 or simplifying 4 X = 80
X must be 20
So B is X so B is 20
and A is 3X so A must be 60
=====================
YOu could let A = X
Then since A is 3 times B then B is 1/3 A or 1/3 X
so 1 1/3 X = 80
4/3 X = 80 so X = 60 and X represents the value of A
and B is 1/3 of A so it's 20
Either way, you get the answer, of course