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post.scala

case class Deposit(period: Int, interest: Double) {
  def gain(amount: Double, duration: Int) = {
    val interestPerPeriod = interest * (period / 12.0)
    val fullPeriods       = duration / period
    val finalBalance      = amount * math.pow(1 + interestPerPeriod, fullPeriods)

    finalBalance - amount
  }
}