12. (5 points) Two candles of the same thickness and material have a length ratio of $21: 16$. They start burning at the same time. After 18 minutes, the length ratio of the longer candle to the shorter one is 15: 11. The longer candle can still burn for $\qquad$ minutes.
【Solution】Solution: According to the analysis, $21-16=5, 15-11=4$, then: the ratio of the two segments of the candle is
21: $16=(21 \times 4):(16 \times 4)=84: 64$;
After 18 minutes: $15: 11=(15 \times 5):(11 \times 5)=75: 55$,
The longer candle burned: $84-75=9$ parts, and the shorter segment of the candle also burned: $64-55=9$ parts, each part burned for: $18 \div 9=2$ minutes,

The longer candle can still burn for: $75 \times 2=150$ minutes.
Therefore, the answer is: 150.