There is no decimal point. Integer division can be useful, but not when trying to learn basic math. Also, "teens and adults" would probably need to practice things like x^2-4x+4=0, or d/dt(-9.81t^2). This app only does the four basic functions, which literally any calculator can do for you.
Additionally, quick inputs are impossible. Try typing 11 at a reasonable speed and you get 1.
Yankeexpress about Math Duel: Algebra Practice