Let's first find how much it is for one pen. So we'll divide the price of 5 of them by 5:
[tex]\frac{6.25}{5} = 1.25[/tex]
Now we'll divide 17.50 by 1.25 to see how many we can buy with $17.50:
[tex]\frac{17.5}{1.25} = 14[/tex]
So your final answer is 14 pens.