Farmer Xavier has 4 roosters. The mass of Rooster M is the same as the average mass of the 4 roosters. Rooster N weighs 2 kg less than the average mass of the 4 roosters. The mass of Rooster P is 8 kg more than the average mass of the 4 roosters. The mass of Rooster Q 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 M = 1 u
Mass of Rooster N = 1 u - 2
Mass of Rooster P = 1 u + 8
Mass of Rooster Q = 7 kg

Total mass of the 4 roosters
= 1 u + (1 u - 2) + (1 u + 8) + 7 
= 3 u - 2 + 8 + 7
= 3 u + 13

4 u = 3 u + 13
4 u - 3 u = 13

1 u = 13

Total mass of the 4 roosters
= 4 u
= 4 x 13
= 52 kg

Answer(s): 52 kg