If the number of sesame to plain bagels is 1:3, that means that for every 3 plain bagels, there is 1 sesame bagel. So if there were 4 bagels total, with 3 being plain and 1 being sesame, then the fraction of the total amount of bagels represented by the amount of plain bagels there are would be 3/4.
3/4 written as a decimal would be 0.75, or 75%.
Hope that helped =)
