Farmer Lee has 4 roosters. The mass of Rooster E is the same as the average mass of the 4 roosters. Rooster F weighs 1 kg less than the average mass of the 4 roosters. The mass of Rooster G is 4 kg more than the average mass of the 4 roosters. The mass of Rooster H 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 E = 1 u
Mass of Rooster F = 1 u - 1
Mass of Rooster G = 1 u + 4
Mass of Rooster H = 7 kg

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

4 u = 3 u + 10
4 u - 3 u = 10

1 u = 10

Total mass of the 4 roosters
= 4 u
= 4 x 10
= 40 kg

Answer(s): 40 kg