この場合は、分かりやすくするため「まぐれで」を「運が良くて」ととらえ、"lucky" や "luck"に置き換えて訳すといいでしょう。上述の例文では、さらに「(試験問題の)半分」を口語表現で "half of them" と訳し、
"I got lucky for more than half of them"
→「運が良くて半分とれた 」
"Half of them were pure luck"
→「半分は運が良かっただけ」
という形でニュアンスを伝えています。また、２つ目の例文で行っているようにluckの前にpureを置くことで、「ただ単に運が良かった」または「運が良かっただけ」という風に強調が出来ます。
A lucky guess is when you don't know the answer, you guess and you get it right. You are lucky
Pure luck involves luck and nothing else. No skill or knowledge
A lucky guess は答えがわからないのに、たまたま正解したときに言います。幸運ですね！
Pure luck は、能力や知識と関係なく、運以外の何物でもない、というときに使います。
1. "Just lucky" is the same as saying "pure luck". The word "just" signifies that the only factor involved in getting those answers correct was luck.
2. I suggest the word "guess". If you are guessing then it means that you are not sure. If you get the answer right, it implies that you were lucky.
3. My third suggestion includes the phrasal verb "based on".
When something is based on luck, it means that the answers didn't involve any skill or intellectual attribution, on the contrary, it was solely due to fortune or luck.
I had an exam, I did not study for, but I got quite a high score. We call that lucky.
"I was lucky, that I knew more than half the answers."
"I passed my test, by pure luck."
テストがあったんだ。勉強してないけど、いい点とれた。こういうのをluckyと言います。
例
I was lucky, that I knew more than half the answers."
半分以上の答えを知っていてラッキーだった。
"I passed my test, by pure luck."
純粋な運だけでテストをパスした。
"Half my answers were pure luck" explains that half of you correct answer were just because you were lucky.
"I guessed most of these." means that more than half of the answers you did know so you just guessed.
"Guessed" means to take a chance.
Half my answers were pure luck
正解の半分は、ただ運が良かったということを説明しています。
I guessed most of these.
半分以上の答えはカンで正解したということです。
Guessedは、一か八かやってみるということです。
I must have got about fifty percent of them right by sheer luck!
I was dead lucky and got about half the answers right by guesswork!
Some people have talent and natural ability in things, while others have to struggle relentlessly just to keep up with the pacesetters of their group. And some people are just dead lucky!
Sheer luck = this means 'total luck.' You were extremely lucky to have guessed the right answers in so many cases.
Dead lucky = really lucky
ある事に才能があったり、天与の才能持っていたリする人たちがいますよね。
その一方で絶え間なくもがいたり、グループの先行する人に追いつこうとしてしたりする人もいます。
そして、めっちゃ幸運な人もいます！
Sheer luck = これは 'total luck.'（全くの幸運）と言う意味です。
'You were extremely lucky to have guessed the right answers in so many cases.'
(あなたはすごくラッキーでしたね、感で多くの正解を当てることが出来て！)
Dead lucky = めっちゃ幸運！
I never studied but got half the paper right out of luck.
I got lucky and got half the paper correct even though i never studied for it.
If you want to tell someone that you got lucky and scored 50% on a test you did not study for, you can say something like "I never studied but got half the paper right out of luck." or 'I got lucky and got half the paper correct even though i never studied for it.".
勉強していないのにまぐれでテストで半分点数が取れたことを伝えるときのフレーズです。
"I never studied but got half the paper right out of luck."
（勉強をしなかったけれど、まぐれで半分とれた。）
'I got lucky and got half the paper correct even though i never studied for it."
（全く勉強をしなかったけれど、まぐれで半分正解だった。）
I got really lucky in the test because half the marks I got were lucky guesses
I got lucky in the test. Almost half of the correct answers I got were lucky guesses
When you want to explain that you did not study for a test and are surprised that you got a good mark because you guessed half of the answers you got right; then you may explain this in the following ways:
-I got really lucky in the test because half the marks I got were lucky guesses
-I got lucky in the test. Almost half of the correct answers I got were lucky guesses
テスト勉強をしなくて、カンでといた問題が半分あっていて驚いたことを伝えたいときのフレーズです。
-I got really lucky in the test because half the marks I got were lucky guesses
（テストでまぐれで半分あっていたからすごくラッキーだった。）
-I got lucky in the test. Almost half of the correct answers I got were lucky guesses
（テストでラッキーだった。正解の半分はカンだった。）
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