What are the extremes of the following proportion 3/15 = 12/60

In a proportion there are letters assigned to represent various numbers. a, b, c, and d.

a, b, c, and d are 4 non-zero rational numbers that are terms of proportion.

a/b = c/d  or a:b = c:d

a and d are the end terms or known as the extremes.
b and c are the middle terms or known as the means.

With this information, we can gather that the extremes of the following proportion are 3 and 60.

3/15 = 12/60 ; a/b = c/d
a = 3 ; b = 15 ; c = 12 ; d = 60

3 and 60 are the extremes
15 and 12 are the means

Where is the problems