There are 5 cows and 15 hens. Facts: 1) total of 20 animals 2) cows have 4 legs 3) hens have 2 legs 4) total of 50 legs Using algebra let C be the number of cows and H be the number of hens. Then set up these equations   C +  H =  20 4C + 2H = 50 Alternatively, you could use trial and error. The number of cows has to be less than 13, because 13 or higher would exceed 50 legs.