Farmer Ivan has 4 roosters. The mass of Rooster P is the same as the average mass of the 4 roosters. Rooster Q weighs 1 kg less than the average mass of the 4 roosters. The mass of Rooster R is 7 kg more than the average mass of the 4 roosters. The mass of Rooster S is 11 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 P = 1 u
Mass of Rooster Q = 1 u - 1
Mass of Rooster R = 1 u + 7
Mass of Rooster S = 11 kg

Total mass of the 4 roosters
= 1 u + (1 u - 1) + (1 u + 7) + 11 
= 3 u - 1 + 7 + 11
= 3 u + 17

4 u = 3 u + 17
4 u - 3 u = 17

1 u = 17

Total mass of the 4 roosters
= 4 u
= 4 x 17
= 68 kg

Answer(s): 68 kg