Farmer Reggie has 4 puppies. The mass of Puppy N is the same as the average mass of the 4 puppies. Puppy P weighs 1 kg less than the average mass of the 4 puppies. The mass of Puppy Q is 7 kg more than the average mass of the 4 puppies. The mass of Puppy R is 12 kg. Find the total mass of the 4 puppies.
Let 1 u be the average mass of each puppy.

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

Mass of Puppy N = 1 u
Mass of Puppy P = 1 u - 1
Mass of Puppy Q = 1 u + 7
Mass of Puppy R = 12 kg

Total mass of the 4 puppies
= 1 u + (1 u - 1) + (1 u + 7) + 12 
= 3 u - 1 + 7 + 12
= 3 u + 18

4 u = 3 u + 18
4 u - 3 u = 18

1 u = 18

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

Answer(s): 72 kg