Farmer Cole has 4 roosters. The mass of Rooster W is the same as the average mass of the 4 roosters. Rooster X weighs 1 kg less than the average mass of the 4 roosters. The mass of Rooster Y is 9 kg more than the average mass of the 4 roosters. The mass of Rooster Z is 12 kg. Find the total mass of the 4 roosters.
Let 1 u be the average mass of each rooster.

Total mass of 4 roosters
= 4 x 1 u
= 4 u

Mass of Rooster W = 1 u
Mass of Rooster X = 1 u - 1
Mass of Rooster Y = 1 u + 9
Mass of Rooster Z = 12 kg

Total mass of the 4 roosters
= 1 u + (1 u - 1) + (1 u + 9) + 12 
= 3 u - 1 + 9 + 12
= 3 u + 20

4 u = 3 u + 20
4 u - 3 u = 20

1 u = 20

Total mass of the 4 roosters
= 4 u
= 4 x 20
= 80 kg

Answer(s): 80 kg