Achef cooked eight kilograms of mashed potatoes for a dinner party. if the guests only ate six-eights of the amount he cooked, how much did they eat?

They ate 6 kilos of the 8 kilos because you have to reduce 6/8 to 3/4 and 3/4 of 8 is 6. 

you could also do the method where since you start out with 8 kilos, you have to break it up into 8 parts, and it says you ate 6 of 8 kilos, so its 6 kilos. 

It would take her 20 minutes.