Erin buys a bag of peanuts that weighs 3/4 of a pound. Later that week, the bag is 2/3 full. How much does the bag of peanuts weigh now?

So he started with 3/4 lb and ended up with 2/3 of that so 2/3 of 3/4

'of' can be roughly translated as multiply

so 2/3 times 3/4 = 6/12=1/2 lb left

or you know that there are 3 out of 4 parts pound left so 2 out of 3 parts of nuts is = 2/4 or 1/2 lb