SAT数学每日一练daily practice(2018年3月21日)
- 2018年03月21日10:11 来源:小站整理
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2018年3月21日的SAT数学每日一练如下:
Mathematics>Standard Multiple Choice
Read the following sat test question and thenclick on a button to select your answer.
Lisa gives her little brother Sam a 15-second(sec) head start in a 300-meter(m) race. During the race, Sam runs at an average speed of 5 m/sec and Lisa runs at an average speed of 8 m/sec, notincluding the head start. Which of the following best approximates the number of seconds thathad passed when Lisa caught up to Sam?
以下哪一项最接近Lisa追到山姆时所经过的秒数?
A.5
B.25
C.40
D.55
正确答案:B
详细解析:
EXPLANATION
Choice B is correct. Let t represent the time, in seconds, that Sam spends running, let d represent the distance run, and let r represent the rate at which Sam is moving.
Then, since d equals r t, 5 t represents the total distance, in meters, Sam runs in t seconds.
Since Lisa gives her little brother a 15-second head start, it follows that she runs for t minus 15 seconds at a rate of 8 text m / sec end text.
So, 8 left parenthesis t minus 15 right parenthesis represents the total distance, in meters, Lisa runs in left parenthesis t minus 15 right parenthesis seconds.
If Lisa catches Sam, they would have covered the same distance. Therefore, we can set up the following equation and solve for t:
table attributes columnalign right left end attributes row cell 8 left parenthesis t minus 15 right parenthesis end cell cell equals 5 t end cell row cell 8 t minus 120 end cell cell equals 5 t end cell row cell 3 t end cell cell equals 120 end cell row t cell equals 40 end cell end table
Since t represent the time, in seconds, that Sam spends running, this shows that Sam will run for 40 text seconds end text before his sister catches him.
Since Lisa runs for t minus 15 seconds, it follows that she runs for 25 seconds before catching up with Sam.
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