一ひとつずつ

adverb, noun which may take the genitive case particle 'no'

one by one; one each; one at a time

Used when items are handled individually, distributed one per person/thing, or done sequentially.

See also: ずつ

• お菓か子しを一ひとつずつ配くばった。

  I handed out the snacks one by one.

• 問もん題だいを一ひとつずつ解かい決けつしていきましょう。

  Let's solve the problems one at a time.