Farmer Jack has 4 roosters. The mass of Rooster A is the same as the average mass of the 4 roosters. Rooster B weighs 6 kg less than the average mass of the 4 roosters. The mass of Rooster C is 8 kg more than the average mass of the 4 roosters. The mass of Rooster D is 13 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 A = 1 u
Mass of Rooster B = 1 u - 6
Mass of Rooster C = 1 u + 8
Mass of Rooster D = 13 kg

Total mass of the 4 roosters
= 1 u + (1 u - 6) + (1 u + 8) + 13 
= 3 u - 6 + 8 + 13
= 3 u + 15

4 u = 3 u + 15
4 u - 3 u = 15

1 u = 15

Total mass of the 4 roosters
= 4 u
= 4 x 15
= 60 kg

Answer(s): 60 kg