Farmer Mark has 4 roosters. The mass of Rooster K is the same as the average mass of the 4 roosters. Rooster L weighs 3 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 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 K = 1 u
Mass of Rooster L = 1 u - 3
Mass of Rooster M = 1 u + 10
Mass of Rooster N = 7 kg

Total mass of the 4 roosters
= 1 u + (1 u - 3) + (1 u + 10) + 7 
= 3 u - 3 + 10 + 7
= 3 u + 14

4 u = 3 u + 14
4 u - 3 u = 14

1 u = 14

Total mass of the 4 roosters
= 4 u
= 4 x 14
= 56 kg

Answer(s): 56 kg