To find the percentage of customers who spent less than $20 at the book sale, we need to know the total number of customers and the number of customers who spent less than $20.
Let's assume that there were 100 customers at the book sale. If we know that 60 of these customers spent less than $20, then the percentage of customers who spent less than $20 can be calculated as follows:
Percentage of customers who spent less than $20 = (Number of customers who spent less than $20 / Total number of customers) x 100%
= (60 / 100) x 100%
= 60%
Therefore, 60% of the customers at the book sale spent less than $20.
Note that if we had a different total number of customers or a different number of customers who spent less than $20, the percentage would be different.