一遍(いっぺん)に(all) at once, at the same time, in one go, in one sitting, right away, in an instant

いっぺんに水(みず)を運(はこ)んだ。

I carried the water all at once.

いっぺんに５個(こ)の薬(くすり)を飲(の)んだ。

I took five pills at once.

いっぺんに終(お)わらせたいです。

I want to finish it all at once.

「いっぺんに１０匹(ぴき)も釣(つ)れたよ！」たけし：「すご！僕(ぼく)もそっちに行(い)く！」

"I caught 10 fish at once!" Takeshi: "Awesome! I'll come over there too!"

母：「家にはいっぺんに２０人も入れないよ。」娘：「でも、みんなでお泊まり会をしたいの。」

Mother: "We can't fit 20 people in the house at once." Daughter: "But I want to have a sleepover with everyone."