hmmm...seems like we are missing some information here. Without more information, the only way to write the equation is to make some assumptions. When he rented the van, was he charged a flat rate per day or for the miles driven over the 3 day span?
Let's assume: he is charged x dollars per mile driven. That would make the number of days he rented irrelevant. So, miles driven * (x dollars/mile driven) = $104.49. Assume x = rate he is charged per mile ( $/mile).