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*n! means n x (n – 1) x … x 3 x 2 x 1*

*For example, 10! = 10 x 9 x … x 3 x 2 x 1 = 3628800,*

*and the sum of the digits in the number 10! is 3 + 6 + 2 + 8 + 8 + 0 + 0 = 27.*

*Find the sum of the digits in the number 100!*

The approach to this problem is quite similar to problem 16 which we can see here. We just need to deal with a really large number here. So, GMP again to the rescue.

library(gmp)

x <- factorialZ(100)

sum(as.numeric(unlist(strsplit(as.character(x), split=””))))

# as.character(x) coverts the big number into a string

# strsplit() extracts individual elements of the string. This can be ensured by using the split = “” argument, which splits the string one element at a time

# strsplit() returns a list. To access the elements of this list, use unlist()

# convert the individual characters to numeric using as.numeric()

# sum the elements

Ans: 648

To

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