Farmer Luis has 4 roosters. The mass of Rooster N is the same as the average mass of the 4 roosters. Rooster P weighs 4 kg less than the average mass of the 4 roosters. The mass of Rooster Q is 8 kg more than the average mass of the 4 roosters. The mass of Rooster R 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 N = 1 u
Mass of Rooster P = 1 u - 4
Mass of Rooster Q = 1 u + 8
Mass of Rooster R = 5 kg

Total mass of the 4 roosters
= 1 u + (1 u - 4) + (1 u + 8) + 5 
= 3 u - 4 + 8 + 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