Farmer George has 4 rabbits. The mass of Rabbit P is the same as the average mass of the 4 rabbits. Rabbit Q weighs 1 kg less than the average mass of the 4 rabbits. The mass of Rabbit R is 6 kg more than the average mass of the 4 rabbits. The mass of Rabbit S is 12 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 P = 1 u
Mass of Rabbit Q = 1 u - 1
Mass of Rabbit R = 1 u + 6
Mass of Rabbit S = 12 kg

Total mass of the 4 rabbits
= 1 u + (1 u - 1) + (1 u + 6) + 12 
= 3 u - 1 + 6 + 12
= 3 u + 17

4 u = 3 u + 17
4 u - 3 u = 17

1 u = 17

Total mass of the 4 rabbits
= 4 u
= 4 x 17
= 68 kg

Answer(s): 68 kg