Your number is highlighted. Tap anywhere on the screen to advance to the next number. If you pick a divisor (in either direction) then you advance. If you pick anything else it is game over. And if you aren't quick enough you will run out of time.
Depends on the difficulty setting and highest number divided by the lowest number you have moved between.
From the App Store, for iPhone and iPad. For free.
I have made another iOS game called Dr Porter's Memory and Number Trainer.