You can write the proportion as 2 equal fractions or as two equal ratios.
[tex] \frac{12}{10} = \frac{x}{15} [/tex] 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.