Okay so, let's analyze the question first.
We're looking for the cost of the 10 pens that Nicholas bought, right? If he bought 5 for $3, how much do you think the 10 pens will cost?
If you multiply 3 by 2, because he'll buy twice as much as he bought the first time, it'll equal 6.
So..
5 pens = $3
10 pens = $6
If he buys 15, 20, or 25 pens, it'll equal..
5 pens = $3
10 pens = $6
15 pens = $9
20 pens = $12
25 pens = $15
We're basically counting off by 3 to know the cost. I hope this helps!