Pigeons and hedgehogs 36 heads and 100 feet?

Answer 1

P = Number of Pigeons
H = Number of Hedgehogs

A Pigeon (P) has 1 head and 2 feet
A hedgehog (H) has 1 head and 4 feet

Thus the total number of heads (36) = P + H = 36, and
the total number of feet = 2P + 4H = 100

Rearranging the first expression gives
P = 36 - H
Substituting this into the second expression gives

2(36 - H) + 4H = 100
72 - 2H +4H = 100, simplyfying and taking 72 off both sides gives
2H + 72 - 72 = 100 - 72, thus
2H = 28, thus
H = 14 (i.e. there are 14 hedgehogs)

From the number of heads we know there are 36 creatures (P + H = 36), thus there must be 22 Pigeons.