3part problem:
jen bought some sesame bagels and some plain bagels. the ratio of the number of sesame bagels to the number of plain bagels is 1: 3.
a)what fraction of the bagels are plain?
b)what percent of the bagels are plain?
c)if jill bought 2 dozen bagels, how many of each type of bagel did she buy?

The information's given in the question has to be carefully checked before starting to solve the parts of the problem that are given.
Ratio of the number of sesame bagels to the number of plain bagels = 1:3
Then
a) Fraction of the bagels that is plain = 3/4
b) Percentage of bagels that is plain = (3/4) * 100
                                                           = 3 * 25
                                                           = 75 percent
c) Number of bagels bought by Jill = 2 dozen
                                                         = (2 * 12)
                                                         = 24
Let us assume the common ratio to be = x
Then
x + 3x = 24
4x = 24
x = 24/4
  = 6
Then
The number of sesame bagels bought by Jill = 6
The number of plain bagels bought by Jill = 3 * 6
                                                                   = 18
So Jill bought 6 sesame bagels and 18 plain bagels in total.