Verified answer

"Toita" is grammatically correct.

Firstly, 「解く」(toku: to solve) and 「解ける」(tokeru: to be solvable/solved) are the different verbs for the different usages. For example:

●あなたはその問題を解く。(You solve the question.)

○その問題は解けました。（The problem was/has been solved.)

In this case, the verb that must be chosen is 「解く」, not 「解ける」.

Secondly, the basic conjunction for the verb 「解く」 (toku) is as below.

・Imperfective form (mizen kei)：解か-ない (toka-nai)

・Conjunctive form (ren-you kei)：解き-ます (toki-masu)

・End form (syuushi kei)：解く (toku)

・Attributive form (rentai kei)：解く-とき (toku-toki)

・Hypothetical form (katei kei)：解け-ば (toke-ba)

・Imperative form (meirei kei)：解け (toke)

The auxiliary verb 「た」(ta) standing for perfection or the past tense is attached to the conjunctive form. So it should have become 「解きた」(toki-ta).

However, "tokita" is so difficult to pronounce that its sound is changed to "toita". It is a kind of euphony in Japanese language.

In fact, this question is difficult to explain, even to native learners.

解いた = toita ; not tokeita

>> past tense of 解く(toku) meaning "find out", "resolve" or "cipher".

あなたはどのようにしてその問題を解いたのですか。

anata wa donoyounishite sono mondai wo toita no desu ka.

this means

how did you solve this problem?

how did you answer/ find the question?

Toita, past tense of 'toku', with this sentence, sounds like more about exam/test. 'mondai wo toku'= to answer the question

it's toita actually...