def payback_of_investment(investment, cashflows):
    """The payback period refers to the length of time required 
       for an investment to have its initial cost recovered.
       
       >>> payback_of_investment(200.0, [60.0, 60.0, 70.0, 90.0])
       3.1111111111111112
    """
    total, years, cumulative = 0.0, 0, []
    if not cashflows or (sum(cashflows) < investment):
        raise Exception("insufficient cashflows")
    for cashflow in cashflows:
        total += cashflow
        if total < investment:
            years += 1
        cumulative.append(total)
    A = years
    B = investment - cumulative[years-1]
    C = cumulative[years] - cumulative[years-1]
    return A + (B/C)