Thanks a lot, Pete. Sorry for interpreting my intentions in the wrong way, just that I'm a beginner, and a soddy one to say the least.

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// A program to find the sum and difference of two numbers
var firstNum;
var secondNum;
firstNum = window.prompt('Please enter the first number', '');
firstNum = parseFloat(firstNum);
secondNum = window.prompt('Please enter the second number', '');
secondNum = parseFloat(secondNum);
write('The sum of ' + (firstnum) + ' and ' + (secondNum) + 'is' + (firstNum + secondNum) + '<BR>');
write('The difference of ' + (firstNum) + ' and ' + (secondNum) + ' is ' + (firstNum - secondNum) + '<BR>');

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Oh, and I'm a Mac.