50 g
Lets simplify definitions: Key-k, Coin- c, Toy Soldier- s, Apple- a, Lemon- l, Plum- pl, pen- p
We have:
2k+2c+3s=1a+1s+1l=100 g----> (1)
1k+1c+1s+1pl=50 g -----> (2)
1l+1a+1pl=1c+1k+1s+1p----> (3)
1p=?
From (1) & (2), 2k+2c+2s=100-1s & 2k+2c+2s=100-2pl => 1s=2pl
From (2) & (3), 1l+1a+1pl=50-1pl+1p => 1l+1a+2pl=50+1p => 1l+1a+1s=50+1p => 100=50+1p => 1p=50 g
1p=50 g