ProjectEuler.1.rb

leadingMultiple = 0
multiples = {}
# first do multiples of 3:
while leadingMultiple < 1000 do
  multiples[leadingMultiple] = :present;
  leadingMultiple += 3
end

# reset to zero
leadingMultiple = 0
# and now do multiples of 5:
while leadingMultiple < 1000 do
  multiples[leadingMultiple] = :present;
  leadingMultiple += 5
end

puts multiples.keys.inject(:+)