According to the new study, to be published in the journal Nature Communications, it's when failure occurs 15% of the time. Put another way, it's when the right answer is given 85% of the time.

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A study by team of psychology and cognitive science researchers from University of Arizona, Brown University, University of California, Los Angeles and Princeton, came up with the 85% rule, said that the “zone of proximal difficulty” is where, you will be maximising your learning.

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For humans, the 85% Rule would mostly likely apply to perceptual learning, in which we gradually learn through experience and examples.

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“If I give really easy examples, you get 100% right all the time and there's nothing left to learn. If I give really hard examples, you'll be 50% correct and still not learning anything new, whereas if I give you something in between, you can be at this sweet spot where you are getting the most information from each particular example.”, said the lead author of the study.

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This doesn’t mean that students should aim for a B in school, just that there might be lessons for more complicated forms of learning that are worth exploring.

"If you are taking classes that are too easy and acing them all the time, then you probably aren't getting as much out of a class as someone who's struggling but managing to keep up”.