To find the percentage of sesame seed bagels that were sold, we need to first calculate the total number of sesame seed bagels that were sold. We can do this by multiplying the number of packages of sesame seed bagels sold by the number of bagels in each package.
Let's assume that each package contains 6 bagels. Then, the total number of sesame seed bagels sold is 15 packages * 6 bagels/package = <<15*6=90>>90 bagels.
Next, we need to divide the number of sesame seed bagels sold by the total number of bagels sold and multiply the result by 100% to express the answer as a percentage. In this case, the total number of bagels sold is 200 packages * 6 bagels/package = <<200*6=1200>>1200 bagels.
So, the percentage of sesame seed bagels sold is 90 bagels / 1200 bagels * 100% = 7.5%.