F+C = 280 (1)
(1 + 1/5)C + (1 + 3/20)F = 328 (2)
now simplify and solve them simultaneously.
F + C = 280 ------ (1)
1320 F + 115 C = 328 ------ (2)
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320F + 15C = 48
3F + 4C = 960
4F + 4C = 1120
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F = 160 C = 280 - 160 = 120