Let's be sure of what we have. From the US DOE: "E85 (or flex fuel) is a term that refers to high-level ethanol-gasoline blends containing 51% to 83% ethanol, depending on geography and season". So unless you do an assay, you don't really know what you have exactly.
Assuming that you are trying to make this liquid suitable for fuel in engines rated for 10%, you could assume that its the "worst case": 83% ethanol. I assume volume percent, and ignore any volume change of mixing*.
Basis: 25 gallons of 83% ethanol. That is 20.75 gallons of pure ethanol, and 4.25 gallons of ethanol-free gasoline. We want 10% ethanol, so the final total amount of fuel, T, times ten percent equals 20.75 gallons. Capisce? Therefore, T equals 20.75 gallons /0.10, or 207.5 gallons. While trivial, you did start wtih 4.25 gallons of gasoline in the mix, so you need to add 203.25 gallons of pure gasoline. Or a bit more than 8 gallons of pure gasoline (zero alcohol) per gallon of E85. Make it simple: Dilute 9:1.
As others have pointed out, you can't dilute 83% (or 51% for that matter) with 10% to get down to 10%. You'll always have more than 10% ethanol. You need to find ethanol-free gasoline to do this dilution.
Unless you have a source of ethanol-free gasoline, it may be easier to find someone using a flexible fuel vehicle and give or sell the stuff to them.
*Volume change of mixing is when, for example, you mix a couple of ounces of 190 proof (98% ethanol) with a couple ounces of water and stir, and get less than four ounces of the mix.