If each term in the sum a1+a2+a3+.....+an is either 7 or 77…

If each term in the sum a1+a2+a3+.....+an is either 7 or 77 and the sum equals 343, which of the following could be equal to n?

This problem repeats another one in the library (at most with different numbers), so it's left out of browsing.

AQuA-RAT — Algebra Question Answering with Rationales (Ling et al., DeepMind, 2017) · train #656