one digit +-x÷

This App "one digit +-x÷" is one digit arithmetic training App. And this is a searching game and speed puzzle game, too.
This App has four categories, addition:"one digit +" subtraction:"one digit -" multiplication:"one digit x" and division:"one digit ÷". Each category has 9 questions on nine stages, random 81 questions.
"one digit +" : from "1 + 1 = 2" to "9 + 9 = 18".
"one digit -" : from "2 - 1 = 1" to "18 - 9 = 9".
"one digit x" : from "1 x 1 = 1" to "9 x 9 = 81".
"one digit ÷" : from "1 ÷ 1 = 1" to "81 ÷ 9 = 9".