He have only 86 cows and dont have birds. let suppose he have x number of cows and y number of birds. then 2x+ 2y=172 (because each have two eyes) 4x+ 2y= 344 (because cows have 4 legs and birds have 2 legs) so x=86 and y= 0.
There are 86 cows only. There are 86 pairs of eyes, so there are 86 animals/birds. To make it 344 legs there should be 86 cows.
There are 12 animals in a farm. Some are Cows The rest are Chickens. There are 28 animal legs in all. How many Cows are there ? How many Chickens are there ?