Published on Sunday, 3rd May 2020, 05:00 am; Solved by 509;
Difficulty rating: 25%

Problem 714

We call a natural number a duodigit if its decimal representation uses no more than two different digits. For example, $12$, $110$ and $33333$ are duodigits, while $102$ is not.
It can be shown that every natural number has duodigit multiples. Let $d(n)$ be the smallest (positive) multiple of the number $n$ that happens to be a duodigit. For example, $d(12)=12$, $d(102)=1122$, $d(103)=515$, $d(290)=11011010$ and $d(317)=211122$.

Let $\displaystyle D(k)=\sum_{n=1}^k d(n)$. You are given $D(110)=11\,047$, $D(150)=53\,312$ and $D(500)=29\,570\,988$.

Find $D(50\,000)$. Give your answer in scientific notation rounded to 13 significant digits (12 after the decimal point). If, for example, we had asked for $D(500)$ instead, the answer format would have been 2.957098800000e7.