Could you tell me whether this is a mistake or not? "Placed" could be correct. But I have consulted the seventh and the eight editions of this book - they does not seem to have corrected it. Maybe they insist on this variant?
Maybe it's just a simple typo. The only thing that's missing is the letter d.
I'm confused by the instruction. It tells me to use the past simple in one space and the past continuous in the other, but then it tells me to use the same tense in both spaces this time.