OBMEP 2015 - 2ª Fase - Data - https://www.youtube.com/watch?v=XxceY97NjEU

help.md

1. Think in a two-digit number.
2. Subtract by it's digits sum. Ex.: 13 - (1 + 3).
3. Sum it's digits. Ex.: 13 => 1 + 3.
4. Sum by 4.
5. Multiply by it's reversed digits number. Ex.: 31 * 13.
6. Multiply by 3.
7. Should be equal 1209.

obmep.js

/*
  Run at: https://repl.it/Gle8/1

  1. Pense num número de dois algarismos.
  2. Subtraia a soma de seus algarismos
  3. Calcule a soma dos algarismos do resultado.
  4. Some 4
  5. Multiplique o resultado pelo numero cujos algarismos são o contrário dele (Ex.: 13 * 31)
  6. Multiplique por 3
  = 1209
*/

const {
  compose
, pipe
, range
, all
, equals
, prop
, map
, reverse
, split
, join
, sum
, add
, multiply
, toString
} = require('ramda')

const
  digits =
    compose(map(Number), split(''), toString)
  
, sumDigits =
    compose(sum, digits)

, reverseDigits =
    compose(Number, join(''), reverse, digits)

, solve =
    pipe(
      x => x - sumDigits(x)
    , sumDigits
    , add(4)
    , x => x * reverseDigits(x)
    , multiply(3)
    )

, check =
    all(compose(equals(1209), solve))(range(10)(100))

check