What is the possible answer to this question? Thank you

Amuffin recipe, which yields 12 muffins, calls for cup of milk for every 1 cups of flour. the same recipe calls for cup of coconut for every cup of chopped apple. to yield a batch of 30 muffins, how much flour will be needed in the mix?

How is working backward similar to solving an equation?

Find the domain of the following expression: 1/(√x)-2