SAT数学每日一练daily practice(2018年3月21日)

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2018年3月21日的SAT数学每日一练如下：

MathematicsStandard 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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