Let's have some normal numbers today, i.e. decimal numbers. They can be quite challenging too. This sum, which the NumberWiz app made for me, is not very difficult, but also not trivial:
How do you add, subtract, multiply and divide 1, 2, 5, 9, 25, 75 to get 131.
Hint: Try to get to 140 first.
As usual your solution is welcome in the comments section.
BTW: have a look @ http://proctele.se for some more apps. Check out the Pi-app, which gives you a million Pi-digits in your pocket? They don't weigh anything. Incredible :-)