If jack already has three cans and needs total thirty Cans he has= 3 Cans needed in total = 30 Therefore, cans left to collect = 30-3=27
(Also when seeing a question, u should not get confused by all the data given. Usually the extra data is given to confuse the person reading the question . Just look at what the question asks and then use the data accordingly. Example here the number of Neighbours are unnecessary yet is confuses the reader. Hope this helps.)
