Let's do Mental arithmetic

This app is devised so that you can train mental arithmetic while having fun as a game.
Mental arithmetic does not improve immediately. If you are not good at mental arithmetic, please don't give up immediately and use it for your daily training.
I have prepared a short video so please use it. Video size: 2.1MB

1. About the calculation formula
Addition and subtraction can be selected in this game.
Furthermore, the number of numbers to be calculated will increase every 10 questions. As the number of numbers increases, you have to memorize the result of the calculation before performing the next calculation, which is a good training.

2. Number of calculation problems
In one game, the maximum number of calculation problems is 40.

3. About the time limit
In this game, the remaining time is set.
The remaining time at the start is 60 seconds.
If you answer the question correctly, the remaining time will be added.

4. End of game
The game ends when the remaining time reaches 0 seconds.
If you answer the 40 calculation problems correctly, the game will end.
You can end the game by pressing the end button even during the game.

5. Difficulty
Difficulty is displayed numerically, starting at 1 and increasing up to 20.
If you answer 40 questions correctly, the difficulty level will increase by one level.

6. Reset
You can return the game to the initial state by pressing and holding the reset button.