SOLUTION: A farmer has a combined total of 30 pigs and ducks and there is a combined total of 100 legs. How many of each animal is there?

There are two ways to do it -- with algebra and without algebra.  
I'll do both.  

1.  WITHOUT ALGEBRA:
There are 100 legs, so there are 50 pairs of legs. 

Ducks have one pair of legs and pigs have two pairs of leg.

If all 30 were ducks, there would only be 30 pairs of legs.

But there are 50 pairs of legs, so the extra 20 pairs of legs must
belong to the pigs.  

So there are 20 pigs and since there are 30 animals there are 10 ducks.

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2.  WITH ALGEBRA:

 d +  p = 30
2d + 4p = 100


Solve that system by substitution or elimination and get d=10, p=20

Edwin