I cheesed out on this one, pretty much just did what the problem called for. No magic. Looking through the forums, however, it was pretty much par for the course.

Problem #20

Find the sum of the digits in the number 100!

product, sum = 1, 0

100.times do |i|
  product *= i + 1
end

while product > 0
  sum += (product /= 10) % 10
end

puts sum