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?
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.