Hi RH, I apologize for the delay in responding to your question! If you'd like to reduce this recipe to fit in your 10X15 pan, I'd suggest multiplying the weight of each ingredient by .64 (for the egg weight, we consider one large egg to weight 50g). This should give you a cake recipe that fits in your pan. The baking time will likely be pretty close to the time recommended, but I would check a bit earlier, and reset the timer from there. 

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