It is an application of a simple subtraction.
Digit is incremented each time it is clear, it becomes more and more difficult.
I think what will change each time you challenge an issue that was clear, and play any number of times.
Since it can be used to save time that was clear, it also allows you to compete with friends.
Difficult to calculate from a simple calculation, adults can enjoy from children.
Because it is a stanza in the 10 questions, the problem will be able to play in the gap time.I think its also ideal for training the brain.
How many digits can you calculate?