There is a saying that you
cannot add apples and oranges. If you have 3 apples and 2 oranges do you have 5
apples? No. Do you have 5 oranges? No. You have five fruits, but the number of
apples and oranges has not changed. Similarly, you cannot add real money and
"what they think they paid"....
Keep the first bulb switched on for a few minutes. It gets warm, right? So all you have to do then is ... switch it off, switch another one on, walk into the room with bulbs, touch them and tell which one was switched on as the first one (the warm one) and the others can be easily identified :-)