In a farmyard there are only hens and cows. Altogether the animals have 48 heads and 140 legs. How many cows are there?

Let there be $x$ hens and $y$ cows. Heads: $x+y=48$. Legs: $2x+4y=140$, that is $x+2y=70$. Subtracting: $y=22$, so $x=26$. Check: 26 + 22 = 48 and 52 + 88 = 140. 26 is the number of hens, the swapped unknown; 35 is 140 divided by 4, which counts every animal as a cow; 24 just halves the 48 heads.