An increase in investment spending will always lead to a proportional increase in real GDP, regardless of the marginal propensity to save.

Investment spending is when businesses spend money to buy things like new machines or buildings, which helps them grow and produce more. When businesses invest more, they create jobs and increase demand for materials and services, which can boost the economy. However, the increase in real GDP may not be proportional to the initial increase in investment because it also depends on how much people save versus spend, known as the marginal propensity to save. For example, if a company invests $1 million and people save a lot of that money instead of spending it, the overall increase in GDP might be smaller than expected. Therefore, while investment spending generally helps the economy, the exact impact on GDP can vary based on saving behaviors.