Sum multiples of 3 and 5

# title: Sum multiples of 3 and 5
# source: https://rosettacode.org/wiki/Sum_multiples_of_3_and_5
# category: Rosetta Code
# difficulty: Intro
# summary: Sum the integers below fixed limits that are divisible by three or five.
# tags: popular, math, loops, project-euler
# vibe: 0.2

def sum_divisible_by(divisor, limit)
  count = (limit - 1) / divisor
  divisor * count * (count + 1) / 2
end

def sum_multiples(limit)
  sum_divisible_by(3, limit) + sum_divisible_by(5, limit) - sum_divisible_by(15, limit)
end

def run
  {
    below_ten: sum_multiples(10),
    below_one_hundred: sum_multiples(100),
    below_one_thousand: sum_multiples(1000)
  }
end