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Fido the Dog, Philemon the Cat and $4$ monkeys together weigh $24$ lbs. Fido and one monkey together weigh $11$ lbs. Philemon and $2$ monkeys together weigh $1$ lb less than Fido and one monkey weigh together. Each monkey weighs the same. How much does Philemon weigh? (2011 Math Kangaroo Problem, Level 1-2, Question \#21) 
Overseas Competition->Knowledge Point->Calculation Modules->Basic Concepts of Equation->Equivalent Substitution
We can write their relationships as the equations below:  $$F+P+M+M+M+M=24$$  $$F+M=11$$  $$P+M+M=F+M-1$$  $ $  $$P+M+M=F+M-1$$, so $P+M+M=10$  $$F+P+M+M+M+M=24$$, so $11+10+M=24$, $M=3$, $P=4$ 