please help

Old Mc Donald raises ducks and cows. The animals have a total of 100 heads and 354 feet. How many ducks and how many cows does Old Mc Donald have?

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$x+y=100\\ y=100-x\\ 2x+4y=354\\ 2x+4(100-x)=354\\ 2x+400-4x=354$

$-2x=354-400\\ -2x=-46$

$x=23$

$y=100-x\\ y=100-23$

$y=77$

Old Mac Donald has 77 cows and 23 ducks.

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