def exp_growth(t, r, x_0):
    """
    Function for calculating exponential growth: f(t)=x_0 * (1+r) ** t
    :param t: time
    :param r: growth rate
    :param x_0: Initial value
    :return: f(t)=x_0 * (1+r) ** t
    """
    return x_0 * (1 + r) ** t