Jan buys 12 pens for 10$. each pen costs the same amount of money. write a proportion to find the number of pens jan can buy $15. then solve your proportion for the number of pens.

You can write the proportion as 2 equal fractions or as two equal ratios.
  or   12 pens:$10 = x pens:$15

You can buy x pens for 15 dollars. One way to find x is by cross multiplying the fractions.

10x=12×15
10x=180
-----  -----
10    10 
x=18

You can buy 18 pens for $15.