Farmer Fred has 4 roosters. The mass of Rooster A is the same as the average mass of the 4 roosters. Rooster B weighs 5 kg less than the average mass of the 4 roosters. The mass of Rooster C is 10 kg more than the average mass of the 4 roosters. The mass of Rooster D is 7 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 - 5
Mass of Rooster C = 1 u + 10
Mass of Rooster D = 7 kg

Total mass of the 4 roosters
= 1 u + (1 u - 5) + (1 u + 10) + 7 
= 3 u - 5 + 10 + 7
= 3 u + 12

4 u = 3 u + 12
4 u - 3 u = 12

1 u = 12

Total mass of the 4 roosters
= 4 u
= 4 x 12
= 48 kg

Answer(s): 48 kg