It would be 3 hours because after three hours, the candle that can only burn for 3 hours will have burned out. This leaves the candles that can burn for 4 and 5 hours. I guess if you want to get really specific you could say 3 hours and 59 minutes but I'm not totally sure how your teacher wants the answer
As she wants to keep two candles burning so after3 hours only 2 candles will remain and after 4 hours only one but till 4 hours 2 candles would be burning that what she wanted so its 4 hours
