Farmer Tom has 4 rabbits. The mass of Rabbit B is the same as the average mass of the 4 rabbits. Rabbit C weighs 2 kg less than the average mass of the 4 rabbits. The mass of Rabbit D is 9 kg more than the average mass of the 4 rabbits. The mass of Rabbit E is 11 kg. Find the total mass of the 4 rabbits.
Let 1 u be the average mass of each rabbit.

Total mass of 4 rabbits
= 4 x 1 u
= 4 u

Mass of Rabbit B = 1 u
Mass of Rabbit C = 1 u - 2
Mass of Rabbit D = 1 u + 9
Mass of Rabbit E = 11 kg

Total mass of the 4 rabbits
= 1 u + (1 u - 2) + (1 u + 9) + 11 
= 3 u - 2 + 9 + 11
= 3 u + 18

4 u = 3 u + 18
4 u - 3 u = 18

1 u = 18

Total mass of the 4 rabbits
= 4 u
= 4 x 18
= 72 kg

Answer(s): 72 kg