Is "A political scandal of this kind hasn't taken place since 50 years ago" acceptabl

I had a task to rewrite the sentence "The last political scandal of this kind took place 50 years ago" using "since".
My teacher suggested 3 ways of answering:
1. There has been no political scandal of this kind since the 1950s.
2. A political scandal of this kind hasn't taken place since 50 years ago.
3. It has been 50 years since the last political of this kind took place.
However, I feel that the two first answers are incorrect.
As for the first answer, we don't know the time of the speaking so we can't say that 50 years ago is the 1950s.
Regarding the second, actually I had never seen this form "since... ago" before.
Would you kindly let me know if they are right or wrong?
Thanks much in advance

There hasn't been a political scandal of this kind in 50 years.

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Originally Posted by linhtho0211

I had a task to rewrite the sentence "The last political scandal of this kind took place 50 years ago" using "since".
My teacher suggested 3 ways of answering:

1. There has been no political scandal of this kind since the 1950s.
2. A political scandal of this kind hasn't taken place since 50 years ago.
3. It has been 50 years since the last political scandal of this kind took place.

However, I feel that the two first answers are incorrect.
As for the first answer, we don't know the time of the speaking so we can't say that 50 years ago is the 1950s. In a sense, you are right. So choose your date of statement [maybe today] and put in the decade that will work: "...since the 1970s/1980s".
Regarding the second, actually I had never seen this form "since... ago" before. It is an incorrect usage.

Would you kindly let me know if they are right or wrong?
Thanks much in advance

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Your instincts are correct, "3. It has been 50 years since the last political [scandal] of this kind took place," is the most idiomatically correct way to rewrite the original sentence using the word since.

Thank you all so much.
For sure and as your advice, my only answer to that question will be sentence 3