The nback task is a method of measuring temporary memory ability (working memory ability) introduced by Kirchner in 1958.

The nBack task was found to have the effect of not only measurement but also improving temporary memory ability itself, and is now being treated as one of brain training.

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Tap the number key matches the one from N steps earlier in the sequence.

In 1'Back
1 task1: count & memorize
2 task2: count & memorize
3 __keyboard__: answer for task1
4 task3: count & memorize
5 __keyboard__: answer for task2
6 task4: count & memorize
:

In 2'Back
1 task1: count & memorize
2 task2: count & memorize
3 task3: count & memorize
4 __keyboard__: answer for task1
5 task4: count & memorize
6 __keyboard__: answer for task2
7 task5: count & memorize
:

Please see the following site for how to use this app.
https://android.brain-workout.org/nback/