One of the sentences I found in an online paper and put into my Anki deck is
77年前【ねんまえ】、原爆【げんばく】はなぜ長崎【ながさき】に落【お】とされたのか?
Why was the atomic bomb dropped on Nagasaki 77 years ago?
How is 77 read?
If I google ななじゅうなな, I get "About 1.730.000.000 results"
and Tae Kim says, "七十七 (ななじゅうなな)= 77"
http://www.guidetojapanese.org/numbers.html
However, a google search for しちじゅうしち produces "About 1.010.000.000 results"
and there is a bank called
株式会社七十七銀行(しちじゅうしちぎんこう、英: The 77 Bank, Ltd.)
https://ja.wikipedia.org/wiki/七十七銀行
Can I just pick my favorite? Does it matter?