Farmer Paul has 4 roosters. The mass of Rooster K is the same as the average mass of the 4 roosters. Rooster L weighs 7 kg less than the average mass of the 4 roosters. The mass of Rooster M is 10 kg more than the average mass of the 4 roosters. The mass of Rooster N is 13 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 K = 1 u
Mass of Rooster L = 1 u - 7
Mass of Rooster M = 1 u + 10
Mass of Rooster N = 13 kg

Total mass of the 4 roosters
= 1 u + (1 u - 7) + (1 u + 10) + 13 
= 3 u - 7 + 10 + 13
= 3 u + 16

4 u = 3 u + 16
4 u - 3 u = 16

1 u = 16

Total mass of the 4 roosters
= 4 u
= 4 x 16
= 64 kg

Answer(s): 64 kg