Can you be a little more specific and provide some additional context? The information you have provided is not enough to allow for an evaluation of your circumstances or a response to the question.
What form are you trying to fill out?
Be sure to designate "best answer." If you live in Oregon, you may call me for more detailed advice, 503-650-9662. Please be aware that each answer on this website is based upon the facts, or lack thereof, provided in the question. To be sure you get complete and comprehensive answers, based upon the totality of your situation, contact a local attorney who specializes in the area of law that involves your legal problem. Diane L. Gruber has been practicing law in Oregon for 26 years, specializing in family law, bankruptcy, estate planning and probate. Note: Diane L. Gruber does not represent you until a written fee agreement has been signed by you and Diane L. Gruber, and the fee listed in the agreement has been paid.
It sounds like you are applying for child support enforcement through the DA's office, or possibly completing a schedule of arrears. Either way, if you are unsure of the precise amount that was paid each month, but know that some money was paid, put in "unsure of amount" (which you will have to squeeze in the space allowed). He will either need to prove how much he actually paid, or you will need to reach a compromise when you get to court. The DA won't go after arrears prior to the date you file with them (unless an order for child support already exists), so the past is somewhat irrelevant to them. And from now on, keep a calendar or notebook and keep track of all money you receive. Also helpful to track his contact with the child if this will eventually become a custody situation . . .
Responses are for general information purposes only, and are based on the extremely limited facts given. A consultation with an attorney experienced in the area of law(s) indicated in the question is highly recommended. Information and advice given here should not be relied upon for any final action or decision, as the information is limited by its nature to the question asked and the fact(s) presented in that question. THIS RESPONSE DOES NOT CREATE AN ATTORNEY/CLIENT RELATIONSHIP, particularly considering that the names of the parties are unknown.