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その問題が解けた

was able to solve that problem

→ 其の
phrase Rare #26,611
was able to solve that problem Uses が with 解ける, the potential form of 解く, to indicate that the problem could be solved. The person who solved it is usually understood from context.
JA: 私は難なくその問題が解けた。
EN: I was able to solve that problem without difficulty.

彼はうまくその問題が解けた。

He successfully solved the problem.

彼の教え子の中に、その問題が解けた者は一人もいなかった。

Not a single one of his students could solve the problem.

私は難なくその問題が解けた。

I solved the problem with ease.

楽にその問題が解けた。

I solved that problem easily.