Final answer:
Marcy has 1500 grams of meat but will use only 900 grams, leaving her with 600 grams left over; no additional meat needs to be added.
Step-by-step explanation:
If you are adding more meat to the mixture, the question implies that you need to adjust the quantities of all ingredients accordingly to ensure the recipe remains balanced. However, since the information provided indicates that Marcy has 1500 grams of meat but only requires 900 grams for her recipe, there is no need for her to add more meat. Instead, she will have 600 grams of meat left over. If the context were different and Marcy needed to add more meat to the recipe than originally called for, she would need to increase the amounts of other ingredients proportionally to keep the ratios consistent.