Question 751777
4P+2D=36   i.e. 4 times the number of pigs is the number of pig-feet, 2D = duck-feet

P+D=10  ten animals total. 

Multiply second equation by 4:

4P+4D=40
4P+2D=36   (and subtract first equation.

2D=4

2 ducks, 8 pigs. 10 animals. 

4 duck feet + 32 pig-feet, total 36 feet.