"They had to live each day, for a month, as an old person, with hidden cameras to record how other people reacted to them".
Why is past simple used in the end of this sentence?
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The main verb of that sentence is in the past tense ("had [to live]"). So the subordinate clause in question ("how other people reacted to them") exists within the context of that past narrative describing this experiment.
Naturally the verb telling about people's behavior (in that past situation) would be in the past tense.