Farmer Japheth has 4 rabbits. The mass of Rabbit D is the same as the average mass of the 4 rabbits. Rabbit E weighs 1 kg less than the average mass of the 4 rabbits. The mass of Rabbit F is 9 kg more than the average mass of the 4 rabbits. The mass of Rabbit G is 5 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 D = 1 u
Mass of Rabbit E = 1 u - 1
Mass of Rabbit F = 1 u + 9
Mass of Rabbit G = 5 kg

Total mass of the 4 rabbits
= 1 u + (1 u - 1) + (1 u + 9) + 5 
= 3 u - 1 + 9 + 5
= 3 u + 13

4 u = 3 u + 13
4 u - 3 u = 13

1 u = 13

Total mass of the 4 rabbits
= 4 u
= 4 x 13
= 52 kg

Answer(s): 52 kg