Periods Command
 Periods( <Rate>, <Payment>, <Present Value>, <Future Value (optional)>, <Type (optional)> )

Returns the number of periods for an annuity based on periodic, fixed payments and a fixed interest rate.

<Rate> Interest rate per period.

<Payment> The amount paid in each period.

<Present Value> Total amount that a series of future payments is worth now.

<Future Value (optional)> A cash balance you want to attain after the last payment. If you do not enter a future value, it is assumed to be 0.

<Type (optional)> Indicates when payments are due. If you do not enter a value or you enter 0 the payment is due at the end of the period. If you enter 1 it is due at the beginning of the period.

Periods(10%/12, 200, 400, 10000)
yields a number of payments of 39.98.
Periods(10%/12, 200, 400, 10000, 1)
yields a number of payments of 39.7.
If you make monthly payments on an annual interest rate of 10 percent, use 10%/12 for 
For all arguments, cash paid out is represented by negative numbers and cash received by positive numbers. 
See also Payment, Rate, Present Value and Future Value commands. 