When the government increases spending by 15 billion dollars, it directly injects money into the economy. This spending is typically used for various projects or services that require immediate labor and materials, which can lead to an increase in employment and wages. As businesses and workers receive this money, they are likely to spend it on goods and services, thus driving up aggregate demand further.
In contrast, a 15 billion dollar tax cut gives individuals and businesses more disposable income, but the impact on aggregate demand can vary significantly among recipients. Not everyone will spend the extra money right away; some might save it instead. This means that while a tax cut has the potential to increase disposable income, the actual increase in consumer spending might not be as immediate or robust as with direct government spending.
Additionally, government spending is usually targeted towards specific projects that can stimulate various sectors of the economy, whereas tax cuts depend on consumer behavior, which can be unpredictable. Therefore, while the nominal amounts of spending and tax cuts are the same, their effects on aggregate demand can differ greatly, primarily due to the immediacy and stability of government spending versus the uncertain spending behavior following a tax cut.