Alfa bank. Which quantity of roubles is needed to buy euros for the lowest prices at the moment.

alfabank.js

function digitAt(index, amount) {
  if (index <= 0) {
    throw new TypeError('index must be positive')
  }
  return Math.floor(amount * Math.pow(10, index) % 10)
}

function restAfter(index, amount) {
  const indent = Math.pow(10, index)
  const rest = amount * indent % indent
  const restFloor = Math.floor(rest)
  return amount - Math.floor(amount) - (rest - restFloor)
}

function round(amount, index = 2) {
  const indent = Math.pow(10, index)
  const rest = amount * indent % indent
  const restFloor = Math.floor(rest)
  return Math.floor(amount) + restFloor / indent + (rest - restFloor > 0.5 ? 1 / indent : 0) 
}

function fix(amount, quantity = 2) {
  const indent = Math.pow(10, quantity)
  return Math.floor(amount * indent) / indent
}

function currency(floor = 0, k = 0, l = 0, p = 0) {
  return floor + k / 10 + l / 100 + p / 1000
}

function calcRurAndEur(floor, k, l, p, rate) {
  const eur = currency(floor, k, l, 5)
  const rur = fix(eur * rate)
  if (restAfter(3, rur / rate) <= 0.005) {
    return {
      eur: currency(floor, k, l + 1),
      rur: round(rur + 0.01)
    }
  } 
  return {
    eur: currency(floor, k, l + 1),
    rur: round(rur)
  }
}

function calc(rate, rurMin = 10.00) {
  const eurMin = rurMin / rate
  // Number = floor.klp{other digits of fractional part}
  const floor = Math.floor(eurMin)
  const [k, l, p] = [digitAt(1, eurMin), digitAt(2, eurMin), digitAt(3, eurMin)]
  if (p < 5) {
    return calcRurAndEur(floor, k, l, 5, rate)
  }
  return calcRurAndEur(floor, k, l + 1, 5, rate)
}

function profit(rate, { eur, rur }) {
  return rate - fix(rur / eur)
}

// EXAMPLE
const rate = 76
const answer = calc(rate)
console.log(`${new Date()}
${answer.rur} -> ${answer.eur} (${fix(answer.rur / answer.eur)}) ${rate}
PROFIT: ${fix(profit(rate, answer))} RUR
`)

FORMAT

${DATE}
${RUR_AMOUNT} -> ${EUR_AMOUNT} (${HACK_RATE}) ${REAL_RATE}
PROFIT: ${PROFIT} RUR

Thu Apr 19 2018 12:27:48 GMT+0300 (Russia TZ 2 Standard Time)
10.27 -> 0.14 (73.35) 76
PROFIT: 2.65 RUR

Thu May 17 2018 09:16:43 GMT+0300 (Russia TZ 2 Standard Time)
10.66 -> 0.15 (71.06) 73.5
PROFIT: 2.43 RUR