Hiya Inna. I assume this is a low-level GMAT question? With that assumption, here's how I would go about it.
Remember: ESTIMATE before CALCULATE. So, while you could break down the cost of the soap per ounce, how could you estimate an answer first? Try this on your own. Do you have the answer?
One simple estimation is to compare 64 to 38, and then 3.80 to 1.90.
38 is more than half of 64. 1.9 is half of 3.8. So in this case, you (as the buyer) get more than half the soap for half the price. While this may not follow conventional wisdom of lower prices for bulk items (because of economies of scale), it's nevertheless the right answer here.
Someone doing neither calculations nor estimations but trying to buzz past this answer would get it wrong. But that won't be you! :)
