A pair of pants costs $48, and each shirt costs $36.


From the condition, we can say and write that Derec sold 2x pants and 3x shirts, 
where an integer number x is the common factor.


With it, we can write the revenue equation as

    48*(2x) + 36*(3x) = 3672.


Simplify and solve

    96x     + 108x    = 3672

   204x               = 3672

      x               = 3672/204 = 18.


So, on Friday Derec sold 2x = 2*18 = 36 pants and 3x = 3*18 = 54 shirts.


The total clothing sold was 36+54 = 90 items.


These 90 item comprise 2/3 of the total clothing in his shop.


Nence, the number of unsold items in the shop was 90/2 = 45.


ANSWER.  (a)  On Friday, Derek sold 54 shirts.

         (b)  The number of unsold clothing was 45.

One of the other person's answers is WRONG! The number of shirts sold was NOT 36. 

As  of clothing was sold, number of clothing sold = 2x, and number of unsold clothing = x

As  of clothing sold were pants, number of pants sold = 
If  of clothing sold were pants, then  of clothing sold, or , were shirts

A pair of pants costs $48, and as a shirt cost ¾ a pants’ price, a shirt costs: ¾ * 48 = 36

Since  pants, and  shirts sold for $3,672, we get:

   ------- Dividing by GCF, 12
          
        16x + 18x = 5(306) ----- Multiplying by LCD, 5
              34x = 5(306)
Number of unsold pieces of clothing, or 

Number of shirts sold: