Explain how to evaluate an algebraic expression?

Jen buys sesame bagels and plain bagels. if the ratio of sesame to plain is 1: 3 and jen bought 2 dozen bagels, what percent of bagels are plain? how many bagels are plain? suppose each plain bagel has 0.53 gram of salt, and each sesame bagel has 0.65 gram. use a percent to compare the amount of salt in a plain bagel to that in a sesame bagel.