Answer
$2000$
Work Step by Step
The total flies($20000$) can be represented by the sum of the infinite geometric series with first term $a_1$ and common ratio $r=0.9$. An infinite geometric series converges if and only if $|r|\lt1$, where $r$ is the common ratio. If it converges, then it equals $\frac{a_1}{1-r}$ where $a_1$ is the first term.
Here $0.9\lt1$.
Hence the sum: $20000=\dfrac{a_1}{1-0.9}\\20000=\dfrac{a_1}{0.1}\\20000=10a_1\\2000=a_1$
Thus the answer is $2000$
