[問題]GWD 07-13

[ruruki]

Q13:
Is the number of seconds required to travel d1 feet at r1 feet per second greater than the number of seconds required to travel d2 feet at r2 feet per second?
(1) d1 is 30 greater than d2.
(2) r1 is 30 greater than r2.
答案是E

我想不太通~有誰可以幫我解答一下呢
謝謝溜~~^^

[JS]

Distance = Rate x Time and set t1 and t2 as the time required for travel the distance of d1 and d2 . so we can set the equations
d1= r1 x t1 and
d2= r2 x t2

from statement 1 we know that d1 = d2+30
so d2 +30= r1 x t1
and d2 = r2 x t2 we can not determine the answer so answer A and D are out
from statement 2 we know that r1 = r2 +30
so d1 = (r2 + 30) x t1
d2 = r2 x t2 we cant figure out the answer either So B is out too
now put two statements together
d2+30 = (r2 + 30) x t1
d2 = r2 x t2 which make

t1 = (d2+30) / (r2+30)
t2 = d2 / r2
let us presume that d2 = 10 and r2 =2
we get t2 = 5 and t1 = 5 / 4 which makes t2 > t1
but if we set d2 = 2 and r2 = 10
we get t2 = 1 / 5 and t1 = 4/5 which makes t2 < t1
so we can not determine the answer from both statements together and this makes the answer E the correct choice

[小紅豆]

謝謝js


動作很快

[homaru]

這題以前有人問過嚕~
07-13
http://www.formosamba.com/phpBB3/viewtopic.php?p=8462
http://www.formosamba.com/phpBB3/viewtopic.php?t=1246

下次提問可以先看看置頂連結
thanks

[ruruki]

大感謝^__^懂囉

[SELVICHY]

[quote="JS"]Distance = Rate x Time and set t1 and t2 as the time required for travel the distance of d1 and d2 . so we can set the equations
d1= r1 x t1 and
d2= r2 x t2

from statement 1 we know that d1 = d2+30
so d2 +30= r1 x t1
and d2 = r2 x t2 we can not determine the answer so answer A and D are out
from statement 2 we know that r1 = r2 +30
so d1 = (r2 + 30) x t1
d2 = r2 x t2 we cant figure out the answer either So B is out too
now put two statements together
d2+30 = (r2 + 30) x t1
d2 = r2 x t2 which make

t1 = (d2+30) / (r2+30)
t2 = d2 / r2
let us presume that d2 = 10 and r2 =2
we get t2 = 5 and t1 = 5 / 4 which makes t2 > t1
but if we set d2 = 2 and r2 = 10
we get t2 = 1 / 5 and t1 = 4/5 which makes t2 < t1
so we can not determine the answer from both statements together and this makes the answer E the correct choice[/quote]

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這題沒那麼複雜,如考試這樣討論的話就做不完了
距離等於速率乘以時間
由以上公式知道這是比例問題(因為是乘積關係)
所以條件給你的若不是比例,比如說三倍,一半...等等
只給你多了多少,少了多少,
那就無法比較了.
這樣的觀念不只用在速率問題,
很多比例關係的資充都可以用此法球求出.