Chaos Elementals have a 1/400 chance to drop a rare item (0.25) What is the amount of times I have to defeat these things before I should be able to get the item? (Trying to get the RoD in Terraria)

Me asap the graph shows the relationship between the price of frozen yogurt and the number of ounces of frozen yogurt sold at different stores and restaurants according to the line of best fit about how many ounces of frozen yogurt can someone purchase for $5? 1 cost of frozen yogurt 1.5 2 13 15.5 cost (5) no 2 4 6 8 10 12 14 16 18 20 size (oz)