紅綠燈亮question

wanda46

5) 紅綠燈亮時間那題。題目提到綠燈先亮,但問題是問在5min interval中紅燈亮的maxiam,故應該是165，

--- Why the answer is 165? I think it should be 130.
Because the green light is on at the beginning, then it is switched to yellow light (red light cannot go after green light according to the common sense). So the sequence would be:
15（黄），45，30，15，45，30，15，45，30，15，15（红）

So the total time for red light is 45*3+15=150.

Is my understanding right?

wanda46

//blush
I did not use OG for the math (only use OG SC part). Can you copy the problem here? Thanks a lot.

tongxun

wanda46 ： There is a same kind question in official guide。 you could check it and learn more about the deep meaning。

wanda46

green、yellow、red。在字面上按照这个顺序进行排列，而这样并不代表是绿灯先亮
-- Yes, you are right!

But I think ETS must have different versions of this problem. As for this particular problem: 題目提到綠燈先亮 -- there are two possibilities:
1) The green light starts to flash from the 0 second; and
2) The green light is already on from the 0 second.

For 1), there are only 3 times for red lights, that is 135 seconds in total;
For 2), there could be 3+1/3 times, that is 150 seconds in total (assume green light soon get off from the 0 second.)

Well, better read carefully when we see this problem in real test.

tongxun

绿,黄，红交通灯，亮的持续时间分别是30，15，45。忽略不同颜色的灯交替时的延迟，
在5分钟的interval里，红灯亮得次数最多，红灯一共亮了多久？
[确认] 3*45+30=165.（6次）
---------------------------------
题目的表述应该是green、yellow、red。在字面上按照这个顺序进行排列，而这样并不代表是绿灯先亮。如果绿灯先亮，则题干中的“红灯亮得次数最多”和“求红灯最多时间”是无意义的。因为绿灯先亮，红灯只亮了3次，而绿灯4次。
所以，我认为题目中的意思应该是：有这3个灯，按正常的顺序亮，问红灯亮的最长时间。
这样的话，红灯先亮，则它亮的时间是最长的。
300/90 =3 余30秒。所以红灯的总的亮的时间= 3*45+30=165