Changed my rating from 5 to 2 stars because...

...after a while, and on the same level, it began acting up: switching numbers around, not accepting taps on numbers, had to be tapped multiple times, ending the game abruptly. It stopped being fun or challenging so I ended up deleting it.
My skills were improving & I was playing faster, but I did not want to advance to a more difficult level yet. Instead the game itself became so much more tricky, difficult, non-responsive, it was no longer enjoyable.

Original review:
A fun fast-paced game with multiple levels, it will challenge your mental skills.

NIghtStar44 about GREG - A Mathematical Puzzle Game To Train Your Brain Skills