Which Calculator?

Many schools recommend a certain model of calculator and some will sell you one, making a handy profit for the school, but the model of calculator is not a choice a pupil should have made for them.
The calculator most recommended by schools is the Casio-83ES. This is a natural notation calculator and displays a calculation on the calculator screen to allow it to be easily checked. Square roots are simplified and displayed and answers are given in terms of pi in many cases.
Like all Casio calculators, it will tell you that -3 squared is -9 and 20+10% is 20.1. Both of these are incorrect but many people believe it because they think the calculator cannot be wrong. Simple four function calculators like those found on mobile phones and old fashioned calculators that use reverse polish notation (rpn) do not have this defect. Neither do many calculators made by sharp, whose calculators have all the functionality of Casio calculators.
Casio do however, produce probably the best calculator in the world – the casio FX-991ES PLUS. This is the calculator I recommend to all my students. It has all the natural notation features of the Casio-85ES and also has quadratic and simultaneous equation solvers built in, and a general equation solver which often makes it unnecessary to rearrange equations to find the solution. I reckon for the average student it makes a difference of 5% in the calculator exam, and costs from £13 on amazon.
Believe it or not, this calculator is exam legal, accepted by all exam boards. It does not perform symbolic manipulation, like simplifying x(x+y) – that would rule it out for use in exams.