The ratio of the number of blueberry bagels to plain bagels sold can be found by dividing the number of blueberry bagels sold by the number of plain bagels sold.
Ratio of blueberry bagels to plain bagels = Number of blueberry bagels sold / Number of plain bagels sold
In this case, the number of blueberry bagels sold is 13 and the number of plain bagels sold is 8.
Ratio of blueberry bagels to plain bagels = 13 / 8
Simplifying this ratio by dividing both numbers by their greatest common factor, we get:
Ratio of blueberry bagels to plain bagels = 1.625 : 1
Therefore, the ratio of the number of blueberry bagels to plain bagels sold is 1.625 : 1.