Here डेढ़ बजे is correct. But in another exercise it is incorrect, so DUO please fix this.
I believe baje is correct in adverbial cases (what the call "oblique"?), like "doing sth AT half past one," whereas baja is correct when you're simply saying what time it is now, i.e., "it IS half past one."
What's the difference between
" डेढ़ " and " ढ़ई "
डेढ़ is one and a half so डेढ़ बजे is 'half-past one'.
ढाई (note the correct spelling) is two and a half so ढाई बजे is 'half-past two'.