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Question
find the number of ways iwhich the number 120 can be written as the sum of three positive integers.
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Answer
a + b + c = 120 where a, b, c >= 1
The formula for this is $$^{n-1}C_{r-1}$$ where n = 120 and r = 3
= $$^{119}C_2$$ = 7021
