The experiment by peterson and peterson that required subjects to count backwards after hearing meaningless syllables followed by a number demonstrates the

The answer to this question is  importance of rehearsal for short-term memory
This experiment found that The longer the interval delay, the fewer syllables. Participants on average can recall 80% of syllables after a 3 seconds delay. From this result, we can draw a conclusion that short term memory is limited  when rehearsal is prevented

What was the length to begin with?