Farmer Tim has 4 roosters. The mass of Rooster B is the same as the average mass of the 4 roosters. Rooster C weighs 6 kg less than the average mass of the 4 roosters. The mass of Rooster D is 10 kg more than the average mass of the 4 roosters. The mass of Rooster E is 5 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 B = 1 u
Mass of Rooster C = 1 u - 6
Mass of Rooster D = 1 u + 10
Mass of Rooster E = 5 kg

Total mass of the 4 roosters
= 1 u + (1 u - 6) + (1 u + 10) + 5 
= 3 u - 6 + 10 + 5
= 3 u + 9

4 u = 3 u + 9
4 u - 3 u = 9

1 u = 9

Total mass of the 4 roosters
= 4 u
= 4 x 9
= 36 kg

Answer(s): 36 kg