# The ratio of my blue pens to my black pens is 7:2; I add four more black pens to my collection. How many blue pens must I add to maintain the same ratio of blue to black?

$11 : 6$
$7 : 2$
$7 - 2 = 5$, so you would have to maintain the difference of $5$. If you added $4$ more black pens that would equal to $6$ black pens total with $7$ blue pens. To maintain the difference of $5$ you would need to have $11$ blue pens.