The Number Wall App
The Number Wall App is available for iPhone and iPod (requiring iOS6 at least).
It offers you a lot more possibilities than the mere addition
of numbers:
 Immediate verification of results
You get immediate feedback whether you calculated correctly or not: the
background of the field in question will be colored red if the result is not correct.
 Calculation with positive and negative numbers
You may choose to calculate not only with positive numbers but with negative ones as
well (what's called "whole numbers"):
7+(9)=2 or
3⋅(7)=21
 Fractional calculus
Add fractions:
3/7+4/5=(15+28)/35=43/35
It is not necessary to simplify a fraction (if possible) for the result
to be accepted as correct.
 Modular arithmetic
This means that you don't calculate with the numbers themselves but
with their remainder after division by a given number (the divisor).
In fact you do it all the time: 5 hours after 10 a.m. is the
15^{th} hour of the day or 3 p.m.; that is the remainder
of 15 divided by 12.
Modular arithmetic is indicated in the Number Wall by e.g.
+/17 showing in the background of an empty field.
Which, in this case, means you should add remainders of numbers
divided by 17 (also called "modulo 17"). Then, for example, the result of
2 + 16 is not 18 but
2 + 16 = 1
 Use help
If you don't know the result of, say, 56⋅75
or 7/12+23/45, you can get help by writing
=56⋅75 and =7/12+23/45
respectively into the field in question. The result then will
show up and the background of the field will be colored yellow.
Perhaps you are familiar with these kinds of formulae from spreadsheets (like Excel).
Screenshots:
Due to the great diversity of possible calculations, the Number Wall App
is interesting, challenging and instructive from elementary school age
up to the first years in secondary school.
