Electronic calculator

An electronic calculator having means for enabling a user to input a calculation, characterised in that the calculator has a mode of operation in which, after entering a calculation to be performed by the calculator, the user is required to input additionally an estimate of the result of the calculation and, for at least a predetermined number of attempts, the result of the calculation is displayed by the calculator only if the estimate lies within a predetermined tolerance range of the correct result, the tolerance range being determined by the calculator in dependence upon the nature of the operators, the numerical value of the operands in the calculation and any tolerance range of the operands.

A calculator as claimed in any preceding claim, wherein after a predetermined number of estimates have been entered that lie outside the predetermined tolerance range of the correct result, the calculator displays the correct result of the calculation.

A calculator as claimed in any preceding claim, wherein a measure of the ability of the user in mental arithmetic is derived from analysis of the estimates provided by the user.

A calculator as claimed in any preceding claim, having a second mode of operation in which the user is not required to enter an estimate of the result in order for the result to be displayed and means are provided for switching between the two operating modes.

A calculator as claimed in any preceding claim, wherein the calculator has a further mode of operation for testing the user, in which further mode the calculation operators and operands are set by the calculator.

A calculator as claimed in any preceding claim, wherein the calculation can be inputted into the calculator in the form of an equation having at least one variable and in which the user is required to input additionally at least one estimate of the value of the variable that satisfies the equation before the correct value of the variable is displayed.