Question

find a recipe in a book or online that includes the amount of salt as a fraction. model how to find the amount of salt needed when the recipe is doubled.

Answer 1

Cinnamon roll cakes have salt as a fraction.
¼ tsp. salt
Multiple 1/4 by 2 and you get 2/4 or 1/2 as doubled

Answer 2

Jamaican Black Cake recipe includes, among others, 1/4 teaspoon of salt.

When the recipe is doubled all amounts of ingredients are multiplied by 2. Then the 3 cups of flour are doubled to 2*3 = 6 cups of flour, and the 1/4 teaspoon of salt is doubled to 2*(1/4) = 1/2 teaspoon of salt.