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1 factorial = 1
2 factorial = 1 x 2 = 2
3 factorial = 1 x 2 x 3 = 6
4 factorial = 1 x 2 x 3 x 4 = 24
5 factorial = 1 x 2 x 3 x 4 x 5 = 120...
The triangle numbers are 1, 3, 6, 10, 15, 21...
Forgotten Number
Age 11 to 14
Challenge Level 
1 factorial = 1
2 factorial = 1 x 2 = 2
3 factorial = 1 x 2 x 3 = 6
4 factorial = 1 x 2 x 3 x 4 = 24
5 factorial = 1 x 2 x 3 x 4 x 5 = 120...
The triangle numbers are 1, 3, 6, 10, 15, 21...
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