If Rachel needs 5 kilograms of solution, how many milligrams of solution does she need?

A 5,000,000 milligrams.

explanation

If Rachel needs 5 kilograms of solution, she needs 5,000,000 milligrams of solution.
1 kilogram is equal to 1,000 grams, so 5 kilograms are equal to 5,000 grams.
1 gram is equal to 1,000 milligrams, so 5,000 grams are equal to 5,000,000 milligrams.