A group of goats and ducks have a total of 99 heads and legs among them. There are twice as many ducks as there are goats. How many are there of each?
5G + 3D = 99
2G = D
5G + 6G = 11 G = 99
G = 9 (the number of goats)
D = 18 (the number of ducks)
I used algebra. Without using algebra, you'd have to try different numbers for G, double the number for D, and see what works.