Alyssa used half of the candles, which is 1/2*40 = <<1/2*40=20>>20 candles
The number of candles that remained is 40-20 = <<40-20=20>>20
Chelsea used 70% of the remaining candles, a total of 70/100*20 = <<70/100*20=14>>14 candles.
The total number of candles in the room now is 40 - 20-14 = <<40-20-14=6>>6
#### 6