
For this one, remember that it goes from \(10\) to \(4\).
Therefore, you take \(10 (-\frac{4}{10})^n\) or \(10 (-\frac{2}{5})^n\) instead.
Then, you use the \(\frac{a}{1 - r}\) formula and substitute a for \(10\) and r for \(-\frac{2}{5}\).
You solve and you get the answer in fraction form.