请教几道DS

joywzy

13. If Sara's age is exactly twice Bill's age, what is Sara's age?
(1) Four years ago, Sara's age was exactly 3 times Bill's age.
(2) Eight years from now, Sara's age will be exactly 1.5 times Bill's age.
Ans: D , I choose E

set Sara's age as S, set Bill's age as B
(1)=>S=2B , S-4=3B, => S=-8
(2)=>S=2B, S+8=1.5B => S=-32
age is impossibly negative. So both is insuficient.I choose E.

20. Is5^k   less than 1,000 ?
(1)  5^(k-1)>3,000
(2)  5^(k-1)=5^k-500

Ans: B , i choose D
from (1)=> 5^k>15,000 => 5^k is not less than 1,000 . (1)is sufficent
so i choose D, did I make any mistakes?

14. If Fran jumps straight up off the floor and lands on her feet T seconds later, her feet will reach a max-imum height of 1.22T2 meters above the floor. On one such jump, was Fran off the floor for less than 1 second ?
(1) On her jump Fran’s feet reached a maximum   height of 1 meter above the floor.
(2) On her jump Fran spent more than   second  ascending.

ans: A

I don't understand the whole question. In fact,i am scared about long sentence,even after I read several times, i still don't what does it say.

yvonnezenith

13.(1)=>S=2B , S-4=3（B-4) => S=8 (2)=>S=2B, S+8=1.5(B+8) => S=8 20.agree 14.according to my understanding, the desired answer can be explained as whether T<1. from (1), i have 1=1.22T2? doubt.

joywzy

13, u are right. I made mistakes.

14,can u explain thhis question in Chinesee? Because i can't understand the whole question.What does it mean?

Thank u!

yvonnezenith

我觉得意思是，某某跳起和落下共用时T秒，跳起的最大高度是1.22T2米。问T是否小于1。
（1）跳起的高度是1米
（2）第一跳用时比第二跳长。

1。22T2没错吗？

joywzy

14. If Fran jumps straight up off the floor and lands on her feet T seconds later, her feet will reach a max-imum height of 1.22T^2 meters above the floor. On one such jump, was Fran off the floor for less than 1 second ? (1) On her jump Fran’s feet reached a maximum height of 1 meter above the floor. (2) On her jump Fran spent more than 1/4second ascending. ans: A sorry, i miss some figures. let me try: set the time of jump off as X, set the time of landing as Y (1)=> X+Y= T, 1.22T^2=1 => T^2=根号(1/1.22) =>T<1 =>X<1 (2)=> X+Y=T, X>1/4,无法得出X<1, 我做得对吗?

tongxun

我觉得做得对。

yvonnezenith

1.22T^2=1 => T^2=根号(1/1.22)
还是不太理解。是1.22T的平方吗？还是1.22*T^2？

tongxun

1.22*T^2