Explanation of the Solution

This post is deleted! 
Hi @aaronhma !
Well, from the video, we know that at the start of the year 2110, the foundation has $\(100,000\) left. We also knows that the foundation gives away $\(100,000\) each year! This means that the foundation will give away this exact sum of $\(100,000\) that they have right now, spread evenly over the course of one year. Since 2110 has 365 days, each day, they give away \(\frac{100,000}{365}=\) $\( 273.97\). What might have confused you is that in the solution, it's almost like counting backwards.
Since the foundation gives away $\( 273.97\) per day, the amount they have given away is
\(273.97 \cdot (\text{number of days that have passed in the year)} \).This means that the amount they have left is
\(273.97 \cdot (\text{number of remaining days in the year)} \).So, since there are 31 days remaining, the foundation has
\(273.97\cdot31=\) $\(8493\) left.Hope this helps

This post is deleted!