Let's call the number of blue and black pens, respectively, and . We know that there are 29 pens in total, and no other colors. This means that
i.e. the number of blue and black pens, together, must add up to 29.
Now, we also know that there are 3 more blue than black, which in other words means that the number of blue exceeds the number of black by 3:
We can use this expression to substitute in the first equation:
So, there are 13 black pens. Since the blue are three more than the black, Suzie has 16 blue pens.