# Math Help - Marginal Propensity to save/consume

1. ## Marginal Propensity to save/consume

I am given the marginal propensity to SAVE:
$
\frac{dS}{dI}=\frac{1}{2}-\frac{1.8}{\sqrt[3]{3I^2}}
$

and the first part of the question asks me to: find marginal propensity to CONSUME when total national income (I) is 81 billion

what exactly do I need to do? what is the formula used?

The chapter I am working on is integrals. Thank you!

2. I searched a little online and saw that dS/dI = 1 - dC/dI so if i wanted the marginal propensity to consume could I use dC/dI = 1 + dS/dI?

3. any help is appreciated....thank you