What is a rate that has a denominator of 1 unit is called a rate

A unit rate is a ratio that has a denominator of 1 unit. This term is also called a unit ratio. A unit rate describes how many units of the first type of quantity corresponds to one unit of the second quantity. Examples are 2 feet per second or 5 miles per hour. 

THE MULTIPLE OF 3 ARE: 3 , 6 , 9 ,12
The 1st magazine costs either 3 or 6 or 9 , 12
The 2nd magazine costs 3 or 6 or 9 , 12
The value 12 is to be discarded , then are the sum of tall possibilities to get 12:

mag 1st         mag 2nd          total
             
3                       9                      12
6                       6                      12  
9                       3                      12