A paddock contains some cockatoos (which have 2 legs) and kangaroos (which have 4 legs). The total number of animals is 21 and they have 68 legs in total. Using simultaneous equations, determine how many cockatoos and kangaroos there are in the paddock.
just count the animals and the legs:
c+k = 21
3c+4k = 68
Now solve as usual