Look at the ingredients on the package of Alka-Seltzer. Aside from aspirin, the ingredients are sodium bicarbonate and citric acid. Sodium bicarbonate is baking soda, and citric acid is the acid in citrus fruits such as lemons and oranges.
See if you can add the right amounts of lemon juice and baking soda to the indicator in cup B to produce the exact color that the five drops of Alka-Seltzer solution produced in cup A. Here's how to do it!
Add one drop of lemon juice to the indicator solution in cup B. Swirl the cup. If this does not match the color of the solution in cup A, use your toothpick to add a small amount of baking soda to cup B and swirl again. Keep track of the exact amounts of lemon juice by drops and baking soda by toothpicks you are adding. Observe the colors in the two cups very closely.
When you think you know the right amounts of lemon juice and baking soda, rinse out cups A and B to get ready to test your results. Place 60 mL/2 oz of indicator in cups A and B. Add five drops of Alka-Seltzer solution to cup A as before, and swirl. All at once, add the exact amount of lemon juice and baking soda that you think will match the color in cup A, and swirl.