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Rashi says on Bereshit 25:7 (Sefaria)
מאה שנה ושבעים שנה וחמש שנים A HUNDRED AND SEVENTY FIVE YEARS — (lit, a hundred years, and seventy years and five years) — at the age of one hundred years he was as strong as at seventy, and at the age of seventy he was as five — without sin.
Why did Rashi get the number 70 for the strength? What happened to Avraham Avinu at 70 years old for Rashi to say strong as at 70?
The question here seems to be based on a problem with the translation.
The text of the Rashi in question is:
בן ק' כבן ע' ובן ע' כבן ה' בלא חטא
There is nothing there about "strength".
Rashi simply says that Avraham at the age of 100 was like 70, and at the age of 70 was like 5, without sin.
Compare the translation on Chabad.org:
one hundred years and seventy years and five years: When he was one hundred years old, he was as one who is seventy years old, and when he was seventy years old, he was as one who is five years old, without sin.
As you can see, there is no mention of strength.
The translator on Sefaria seems to have inserted "as strong as", which does not actually appear in the Hebrew text. It is possible that this is the translator's interpretation — e.g. Avraham had the strength to resist the temptation to sin, or some other such explanation, or it might simply be a mistaken translation.
Alternatively, it might be based on a variant text that does have the word לכח in the Hebrew (though Sefaria's Hebrew text does not have this word). In the Mossad HaRav Kook edition of Rashi's commentary, the word לכח appears in brackets with the following footnote:
ליתא בד"ר ולא ידוע לי המקור לפירוש זה
It is not in the first printing, and the source of this explanation is unknown to me.
Here are three manuscripts that do not have the word לכח:
As for what Rashi actually meant with the comparison of 100 to 70, R. David Segal in his supercommentary Divrei Dovid explains that Rashi is only saying one idea. Both comparisons (100 to 70 and 70 to 5) are talking about Avraham being free of sin. (He has an explanation as to why two comparisons are necessary, but I don't think that is necessary for the question at hand).
