From the problem statement, we can write:
C + G + U = 258 (C,G,U) = (China, Great Britain, United States)
C = U - 51
G = U - 54
Substitute for C and G:
(U-51) + (U-54) + U = 258
Combine like terms:
3U - 105 = 258
3U = 363
U = 363/3 = 121 --> C = 70 and G = 67
Check: 121 + 70 + 67 = 258