C is chickens and P is pigs.  Each animal has one head so C + P = 200.
Chickens have two legs, so for legs chickens add 2*C  (2 times the number of chickens.)  Pigs have four legs, so for legs pigs add 4*P.  2C + 4P = 540.
From the first equation solve for C:  C = 200-P.  Substitute that into the second equation.
2(200-P)+4P = 540
400 -2P + 4P = 540
2P = 140
P = 70.
C = 200-70=130.
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