gistfile1.rb

#----------------
# euler1.rb
# Thomson Nguyen
#----------------
# I try to learn ruby by solving Project Euler 
# puzzles. (http://projecteuler.net)
#
# Feel free to take a look at my progress and 
# comment, but don't laugh!
# 
# All code subject to MIT License, blah blah
#
#
# Add all the natural numbers below one thousand that 
# are multiples of 3 or 5.
#
#

# Brute Force

i=0
running=0
while i < 1000
if i.modulo(3)==0 or i.modulo(5)==0
running = running + i
end
i +=1
end
puts running

# Using the inclusion-exclusion principle

def sumform(n)
  return n*(n+1)/2
end
 
def sumMultiples(limit,a)
  return sumform((limit-1)/a) * a
end
 
puts sumMultiples(1000, 3) + sumMultiples(1000, 5) - sumMultiples(1000, 15)