This is key: you are always shooting for a Target Number, which is Prime. Look at the last image. See the 18 in black? Tap that square and it will subtract 1.
At the moment, my high score is 53,100. My highest Prime #: 349. I'm guessing that game took 25 minutes.
Download this game & learn/discover Twin Primes, Cousin Primes, Balanced Primes, Safe Primes, and Triplet Primes.