I am given the marginal propensity to SAVE:

$\displaystyle

\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!