Q...34... NO QUESTION
ONLY SOLUTION

The correct answer is: 2.5 OR 5/2

Given,
The cost of one blue pen  
According to the question, the cost of each red pen is d times that of blue pen.
So, the cost of each red pen = $ 0.60 d
Also,
Number of blue pens bought = 3
Number of red pens bought = 6
Total cost of blue pens = $ (3 * 0.60) = $ 1.80
Total cost of red pens = $ (6 * 0.60d) = $ 3.60d
Thus, total of the pens=  $(1.80 + 3.60d)
It is also given that the total cost of 3 blue pens and 6 red pens is $10.80.
Thus, equating the two quantities, we have

Solving the above equation for d, we have


Dividing by  throughout, we have

Thus, the correct answer is 2.5 or 5/2




.

Note:
This type of question is called a word problem. We need to have simple understanding of total cost of the number of items purchased and multiplication.  There could be many more items and their costs added in the list. This is the standard method of approaching these problems.