Suppose 48 chickens are all chickens, with a total of 48×2 = 96 legs.

Actually, there are 108 legs. Why are 108-96 = 12 legs missing?

Because some of these 48 are rabbits.

Each rabbit has 4-2 more legs than each chicken.

Divide the total number of extra legs by the number of extra legs each rabbit has than the chicken.

That is, 12 ÷ 2 = 6, which is the number of rabbits.

The number of chickens is equal to 48-6 = 42.

Verification: the number of legs of chicken = 42×2 = 84, and that of rabbit = 6×4 = 24.

Total number of legs = 84+24 = 108