A farmer owns a total of 20 ducks and lambs. The ducks and lambs have 64 legs altogether. How many lambs are there?
Let the number of lambs = N
Then the number of ducks = 20 - N
So
No. of ducks x legs each + No. of lambs x legs each = 64 .....so....
(20 -N) * 2 + N ( 4) = 68 simplify
40 - 2N + 4N = 68
2N = 68 - 40
2N = 28
N = 28 / 2 = 14 lambs
+4 
