i have a recipe for 11 sheet cakes (21x12) and i only need a recipe for 1 sheet cake. how do i reduce 3# of flour? etc....

Just make sure that everything is in weight measurements (ounces or grams), then divide everything by 11--to give a little leeway so your recipe isn't too tight(sticking to bowl/whip, etc) you could make it easier by dividing by 10 e.g. 3# flour = 48 oz/10 = 4.8 oz flour.

No, don't confuse her (she isn't a pro)...divide each measurement on a calulator by 11. If you don't understand post the recipe and we can show you.