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There are a number of unusual relationships between certain numbers (as represented in the decimal system). There is not much explanation for them. Just enjoy them and see if your students can find others.

We are going to present pairs of numbers where the product and the sum are reversals of each other. Present them one at a time to your students so that they can really appreciate them.

The two numbers Their product Their sum
9 9 81 18
3 24 72 27
2 47 94 49
2 497 994 499
Ask students if they can find another pair of numbers that exhibits this unusual property. (They may have difficulty with this.)

Here’s another strange relationship? 

1 = 1!
2 = 2!
145 = 1! + 4! + 5!
40
585 = 4! + 0! + 5! + 8! + 5!

(Remember that 0! = 1.)

That appears to be all of this sort that exists, so don’t bother having
students search for more.

? The exclamation mark is called a factorial and represents the product of consecutive integers from 1 to the number before the factorial symbol. That is, n! = 1
2
3
4 · · · · · n ? 2n ? 1n.

Taken From :Math Wonders to inspire teacher and student