"Their names were in the tenth letter."

Translation: Jejich jména byla v desátém dopisu.

[BoneheadBass]

I got this right using "dopise," while the correct answer uses "dopisu." Assuming that both are correct, is "dopisu" preferred? If so, why is that? Thanks!

[AgnusOinas]

It's a matter of personal preference. I wouldn't say one ending is used more frequently than the other. It's the same as "autobusu/autobuse" and many other "hrad"-paradigm words in the locative singular. Some people use one of the two endings consistently, others alternate between them.

In case you want to read a more thorough analysis: http://prirucka.ujc.cas.cz/?slovo=dopis#bref4

[teabeadle]

Why is this "byla" and not "byly"? The subject (jména) is in plural.

[endless_sleeper]

Jméno is neuter. -a is used for neuter plurals.