If we list all the natural numbers below $10$ that are multiples of $3$ or $5$, we get $3, 5, 6$ and $9$. The sum of these multiples is $23$.
Find the sum of all the multiples of $3$ or $5$ below $7^N$, and give your answer as the sum of its digits.
Note: The wording might not be perfect, but the function $g(N)$ that's being asked for below is the result for $7^{2^N}$. This box always gives the function $f(N)$, and you then need to find how to compute $g(N)$, and here $g(N) = f(2^N)$.