Reverse Polish Notation, a k a "postfix notation", is pretty easy. Normally, you might express the number 18 like this:
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| Code: 9 + 9 | |
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where you put the operator in between its arguments. RPN just puts the operator at the end, so 9 + 9 becomes
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| Code: 9 9 + | |
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or 2 x 3 becomes
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| Code: 2 3 x | |
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Easy peasy! So if you're 21 years old, you might express that in RPN by writing something like
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| Code: 20 1 + 7 3 x 30 9 - | |
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or similar. Keep that in mind!