If one pen costs d dollars, what is the cost, in dollars, of 13 pens?

13d

To find the cost of 13 pens, we need to multiply the cost of one pen by the number of pens. In this case, since one pen costs d dollars, we can express the cost of 13 pens as:

13 pens * d dollars/pen = 13d dollars.

So, the cost of 13 pens would be 13d dollars.