Very good. Simulataneous equations does the trick:
W = price of watch
C = price of chain
L = price of locket
W + C + L = 216 — equation 1
W - 3c + L = 0 — equation 2
0.5W + C + L = 0 — equation 3
A 10th grader in a good school in the USA could solve this.
I didnt go to 10th grade in the US
I had an even better British education in NZ