You turn 2 switches "on" and leave 1 switch "off" and wait about a minute. Then enter the room, but just before you enter, turn one switch from "on" to "off". Once in the room, feel the lightbulb - if it is warm, but off, it has to be the last switch you turned off. If it is on, it has to be the switch left on. If it is cold and is off, it has to be the switch you left in the off position.
Mary is not a nurse. The way to solve this riddle, is to consider statements 4, 5, and 6 and create a chart of all possible true and false answers. Next, fill in the chart according to statements 1 through 3. You will discover that there is only one line where only one of statements one, two and three are true. Thus, it is determined that: Statement 4 and 5 are false and statement 6 is true.