# Letters for Numbers Tutorial

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Using variables. We must learn how to use variables in expressions. Just as in algebra we can use letters to represent numbers - they may be added, subtracted and multiplied.

Some integer operators
OperatorMeaningExpression value when x is 7
`+`Addx+310
`-`Minusx-34
`*`Multiplyx*321
`/`Dividex/32
`%`Modulusx%31

## Twice

The program fragment shown defines a function that doubles it's input. When the program is executed we see the result of the function for a handful of values.

The function definition is embedded in a larger program that is responsible for testing the function at values 1, 2, 3, 4, 5 and 6.

Notice what happens if you change the function so that it multiplies by 3 instead of 2.

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## Half

Change the function so that it returns half the input value. This program only accepts an integer result so you can ignore any remainder.

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## Modulus

Change the function so that it returns the remainder when divided by two. For even numbers this is 0, for odd numbers this is 1.

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## Add VAT

Change the function f so that it adds VAT to it's input. VAT is 20%

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