"The dirty dishes are inside the sink."
Translation:הכלים המלוכלכים נמצאים בתוך הכיור.
In English, it's more common, but this is probably to differentiate between ב- & בתוך. (Which to use).
Speaking of which, is there some guidelines of when we can't use bet as a prefix, like here. Because it's much more common to say "in the" ב bowl/sink, etc. Where Duolingo Hebrew uses בתוך ...