Question

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.

Answer 1

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