I don't know why he took part in the walk.I don't know ________he took part in the walk__________.

I don't know why he took part in the walk.
I don't know ________he took part in the walk__________.

I don't know what he took part in the walk for?我不知道他们参加步行的目的是什么?
Why...?==What...for?

what for;
what for:....原因