lets take
weight of traingle as a, weight of square is b and weight of circle is c,
from first and 2nd scales,
a + 2b = 3c ----(1)
a + c = 6b -----(2)
(1)-(2)--> 2b - c = 3c - 6b
2b = c
substituting 2b = c on (1)
a + 2b = 6b
a = 4b
in scale (3), left value is 2b + c
so (1) + (2) -->...