If you are good with decimals it is best to convert it to decimals to make it much simpler. 1 and a quarter is equal to 1.25. You can divide 20 by 1.25 to find the amount for each cookie. If you do that, that should be equal to .0625. From there you can multiply the amount for each cookie (.0625) times every cookie you want to make (.0625*8) to find out your number which should be .5 or one half 1/2.
