fly a balloon app for iPhone and iPad

Lets solve quiz before balloons to fly.
Identify the change of constant value to five balloons.
Through the value of four balloon, guess the value of the ?-balloon.
When the balloon is flied,
if the answer is good, money is fallen from the sky,
but If the answer is wrong, poo is fallen.
They are listed to sequence number using four operation code.(+,-,×,÷)
There are three kinds of difficulty.(easy, normal, hard)
Each difficulty has 30 levels.
Lets play game and improve numerical ability.

How to
1. Identify the change of constant value to four balloons.
2. Guess the value of the ?-balloon.
3. Enter the result of calculation with the number buttons.
4. Fly a balloon with touching Fly button.
5. If you answer wrong, the touch of C button and then re-enter.