Need some clarification with combined hiragana

Hello, I am currently studying some hiragana while doing your audio courses, and a friend of mine found some hiragana worksheets. I've been doing them pretty steadily and I found a question I am a little confused on, so I was wondering if you could help me out. Its in a section where you use a small tsu to make double consonants. Basically its just a simple question of converting the romaji into hiragana. Heres the question: Roppyakuen ___________ I got ろっぴゃくえん but on the answer worksheet it says ろっぽんぎ. Is there any rule I am missing, or is the answer incorrect? Thanks!

quick edit I am pretty sure it says ropponki

Konnichiwa!!　We're glad to hear you are studying Japanese with us! :D I believe the worksheets answer is incorrect. For *Roppyakuen* it is as you wrote: ろっぴゃくえん For *Roppongi *is would be: ろっぽんぎ

Thanks for the help!