So if I understood correctly, this might help you:
Elements elem = doc.select("eta");
Iterator<Element> iterator = elem.iterator();
while(iterator.hasNext()) {
Element etaElement = iterator.next();
Element arrT = etaElement.select("arrT");
Element prdt = etaElement.select("prdt");
// 1. parse the arrT value
// 2. parse the prdt value
// 3. subtract them.
}
Like this you will be able to individually handle each entry. Note the the div.select
is being used for each eta
element and not from the whole document, as your code was doing.