Why is adding three to each digit of a remembered sequence difficult?

Answer & explanation

It requires holding and transforming information under attention

The Add-1 and Add-3 exercises require keeping a sequence of digits in working memory while changing each digit in order. That is much harder than simply recognizing or repeating the digits because temporary storage and mental transformation compete for attention. Adding three is especially demanding, demonstrating how System 2 workload rises as operations become less automatic.