In a MobX-powered e-commerce application, you want to ensure that the cart total is automatically updated whenever an item is added or its quantity is changed. Which approach would you use to achieve this efficient update?

To achieve an efficient update of the cart total in response to changes in items or their quantities, you would use Reactions in MobX. Reactions are triggered automatically when observable data they depend on changes. In this case, a reaction can recalculate the cart total whenever the cart items or their quantities change, ensuring an automatic and efficient update.