Great underrated question....
We have 3 candles that can burn INDIVIDUALLY for a total of ——> 100+ 80 + 60 = 240 minutes.
When any one of the candles is burning individually and NO OTHER candles are burning ——-> that one candle will take away A - number of minutes from the 240 total minutes each candle would burn individually
When EXACTLY 2 candles are burning, we can say that the amount of time that the 2 candles burn together is = B
However, when Exactly 2 candles are burning together, each will individually be taking away minutes from the 240 total———> so for all the B number of minutes that there are exactly 2 candles burning:
(2) * B minutes will be reducing from the 240 total individual minutes of the 3 candles were they to burn separately
Similar logic applies to the case when exactly 3 candles are burning. We can call this amount of time C ————> and during this time of C minute when all 3 candles burn:
(3) * (C) minutes will be reducing from the 240 total individual minutes of the 3 candles were they to burn separately
So the entire 240 total individual minutes is equal to:
240 = A + (2)*(B) + (3)*(C)
We are told that for 40 minutes, exactly 1 of the candles is burning alone ———-> A = 40 minutes
And we are told that for 30 minutes all 3 candles are burning together ———> C = 30 minutes
Plugging these into the equation:
240 = 40 + (2)*(B) + 90
100 = (2)*(B)
B = 55
Answer:
55 minutes exactly 2 candles will be burning.
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