请问这一道很简单的sat数学题
Aclerkaccidentallythrewavaluabledocumentintooneof90trashcans.Itisequallylikelythatthedocumentisinanyofthese90trashcans.Ifexactly15ofthese90trashcansareblue,whatistheprobabilitythatthedocumentwillbeinabluetrashcan?
我觉得答案是1/15但是参考答案是1/6
为什么呢