The farmer has 30 chickens & rabbits altogether. There are 86 legs in total. How many rabbits are there?
Let's assume that the farmer has x chickens and (30-x) rabbits.
Each chicken has 2 legs, so the total number of legs from the chickens is 2x.
Each rabbit has 4 legs, so the total number of legs from the rabbits is 4(30-x).
The total number of legs from both the chickens and rabbits is 2x + 4(30-x).
This equals the total number of legs, which is given as 86.
So, we can set up the equation: 2x + 4(30-x) = 86.
Expanding this equation gives: 2x + 120 - 4x = 86.
Combining like terms gives: -2x + 120 = 86.
Subtracting 120 from both sides gives: -2x = -34.
Dividing both sides by -2 gives: x = 17.
So, there are 17 chickens and 30-17 = 13 rabbits. Answer: \boxed{13}.