Number Stax 4 cert, Houghton Mifflin Harcourt, iOS ****

Number Staxis one of those apps that you should feel good about wasting hours on. It teaches by stealth, sneakily tricking you into improving your maths skills while pretending that it's a puzzle game. The numbers fall as blocks from the top of the screen, similar to Tetris. But instead of making shapes fit or match up jewels, you have to try to figure out what combination of numbers and mathematical symbols will give you the total displayed at the top of the screen. The main aim is to keep it going; like in other games of this type, the stack of blocks will keep pushing higher until it reaches them top of the screen and it's game over. You have a stock of weapons at your disposal. First is an unlimited supply of plus and minus signs to help you solve the puzzles and clear your screen. Second are the actual weapons: bombs, lightning strikes and fire. Each work in a different way, buying you valuable time. On the surface, Number Staxmay be aimed at a younger audience, but it's a great way to keep the grey matter active.