飞机正常飞行15分钟后警示灯开始闪烁。

The plane was flying normally for about 15 minutes before a warning light blinked on.

《柯林斯英汉双解大词典》

一盏灯每晚七点钟亮。

A light went on at seven every evening.

《柯林斯英汉双解大词典》

读者文摘解答：如果是白炽灯，每次只要你离开房间超过5秒钟就要关掉；如果是节能灯，超过15分钟的话一定要记得关。

Reader's Digest Version: Turn off incandescents if you're leaving the room for more than five seconds; CFLs, if you'll be gone at least 15 minutes.

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