class State(val adults: Int, val youngs: Int, val children: Int) {
  override def toString(): String = "(a: " + adults + ", y: " + youngs + ", c: " + children + ")"
}

def evolve(prevState: State): State = {
  val childenPerAdult = 6 / 2 // Six children for each couple
  new State(prevState.adults + prevState.youngs, prevState.children, prevState.adults * childenPerAdult)
}

def stateAtTime(timeZero: State, time: Int): State = {
  time match {
    case 0 => timeZero
    case t => evolve(stateAtTime(timeZero, t - 1))
  }
}

stateAtTime(new State(10, 0, 0), 3*3)