3) B

We know birds have 2 legs and sheep have 4 legs.

Let number of sheep = S
Let number of Birds = B
Total number of animals = 13 = S + B

Total number of sheep legs = 4S
Total number of bird legs = 2B
Total number of legs = 4S + 2B = 36

Using simultaneous equations,

4S + 2B = 36
[S + B = 13] x 2

4S + 2B = 36
2S + 2B = 26

2S = 10
S = 5

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