Three candles can burn for $30$, $40$, and $50$ minutes respectively (but they are not lit at the same time). It is known that the three candles are burning simultaneously for 10 minutes, and only one candle is burning for 20 minutes. Then, the time when exactly two candles are burning simultaneously is $\qquad$ minutes.
The time during which exactly two candles are burning simultaneously is $[(30+40+50)-(10 \times 3+20 \times 1)] \div 2=35$ minutes