I have a short method to solve this type of questions in a second. The trick is Leg÷2 ( - ) mines head but the answer will come out of animals. And you want to get answer of birds then you have to mines(-) from head. Let solve with examples So total no of head is 180 And total no of leg is 420 Find the no of animals and birds So the formula was leg ÷2 - head 420÷2=210-180 = 30 Animal's no is 30 And bird is total no of head mines( - ) Animal's no 180-30 = 150 Thank you If