NumberDuel released for iOS devices!
NumberDuel is a game of mental agility where you simply tap the card with the larger value on it. For each correct answer you receive a point until you reach 21 and the game ends. The stopwatch is running as you play so the faster you go the better your time. But be warned, the penalty for a wrong answer is steep: 3 points off your score!
Easy, right? Well, it's not so tough when you're dealing with whole numbers, but NumberDuel goes way beyond that. Comparing integers makes your brain work a bit harder.
If that's not enough challenge for you, purchase the six additional levels that include Sums, Sum & Difference, Products, Ruler Fractions, Fractions and Decimals. Only those with very strong number sense can tackle these and still beat the clock.