This one is driving me crazy...The explanation in the back of the book doesnt help either...
OG11th ed (DS #131)
Is 5^k less than 1000?
(1) 5^k+1 > 3000
(2) 5^k-1 = 5^k-500
_________________________________________________________
My steps
_________________________________________________________
Step 1: Rephrase the question Is 5^k less than 1000?
possible rephrases
a) 5^k < 1000
b) 5^k < (2x5)^3
c) 5^k < 2^3 5^3
Step 2: What underlying concepts can I use to rephrase/understand the question/statements?
USE THE LAW OF INDICES
" a^m x a^n = a^m+n "
" a^-m = 1/a^m "
Step 3: This is where I get lost...can anyone finish my steps and provide an explanation?
_______________________________________________________
Here's how the OG broke the statements in the back of the book...
_______________________________________________________
OG explanation of Statement (1) 5^k+1 > 3000
step 1 - divide both sides of the given inequality by 5 (*see my note*)
5^k+1 ÷ 5 > 3000 ÷ 5 = 5^k > 600
step 2: Statement 1 - INSUFFICIENT
Although 5^k > 600, it is unknown if 5^k < 1000
** The book confuses me here I thought when you divide you're suppose to change the direction of inequality sign **
___________________________________________
OG explanation of Statement (2) 5^k-1 = 5^k-500
step 1- subtract 5^k from both sides
5^k-1 - 5^k = 5^k - 5^k - 500
step 2 - divide all terms by -1
(5^k ÷ -1) - (5^k-1 ÷ -1) = (-500 ÷ -1)
(5^k) - (5^k-1) = (500) **
** The book confuses me here - am I missing something? I thought when you divide all terms by -1, that 5^k will become - 5^k, that -5^k-1 ÷ -1 will be come +5^k-1
step 3 - property of exponents
5^k-5^k (5^-1) = 500
step 4 - substitute for 5^-1
5^k-5^k(1/5) = 500
step 5 - factor out 5^k
5^k(1-1/5) = 500
step 6 - simplify
5^k(4/5) = 500
step 7 - multiply both sides by 5/4
5^k = 500 (5/4)
5^k = 625 which is less than 1000 - SUFFICIENT
____________________________________
This sure seems like a lot of calculation for a data sufficiency problem. I know my other alternative was to pick a number for k, but that seems like a cop out plus its not getting at the root of the concept behind the problem....
Is there an alternative approach to this problem that still applies the concept that is tested ie, the laws of indices?
Tough one - OG DS Q#131
This topic has expert replies
u shud know 5^1 = 5, 5^2 = 25 5^3 = 125, 5^4 = 625 and 5^5 = 3125
its really easy to calculate for 5
is 5^k < 1000
that means is k < 5
(1) 5^k+1 > 3000
k can be 4/6
Insuff.
(2) 5^k-1 = 5^k-500
the difference between 5^3 and 5^4 is 500
so u can easily determine k = 4
Suff
Choose(b)
its really easy to calculate for 5
is 5^k < 1000
that means is k < 5
(1) 5^k+1 > 3000
k can be 4/6
Insuff.
(2) 5^k-1 = 5^k-500
the difference between 5^3 and 5^4 is 500
so u can easily determine k = 4
Suff
Choose(b)
-
- Legendary Member
- Posts: 661
- Joined: Tue Jul 08, 2008 12:58 pm
- Location: France
- Thanked: 48 times
All you say is very clear but there are a lot of things to answer and many important points you have to work.
Let's try to solve the question first.
Is 5^k less than 1000?
(1) 5^k+1 > 3000
(2) 5^k-1 = 5^k-500
(1)
5^(k+1)>3000
5^k * 5>3000
You divide by 5 both sides, given 5>0 you do not change anything
5^k>600
As OG explains, this is bigger than 600 but we do not know if it is less than 1000.
(2)
I write you what I did, I don't take the OG solution.
5^k-1 = 5^k-500
I just put 500 to the left and 5^(k-1) to the right consequently I change their sign.
500 = 5^k - 5^(k-1)
I factorize
500 = 5^(k-1)*(5 -1)
500 = 5^(k-1)*4
I divide both sides by 4
125=5^(k-1)
5²=25 5^3=125
So, 5^3=125 and we have 125=5^(k-1)
Then, 3=k-1, then k=4
Now, we can answer the question "Is 5^k less than 1000?"
5^4=625<1000
Hence, B is sufficient
Let's try to solve the question first.
Is 5^k less than 1000?
(1) 5^k+1 > 3000
(2) 5^k-1 = 5^k-500
(1)
5^(k+1)>3000
5^k * 5>3000
You divide by 5 both sides, given 5>0 you do not change anything
5^k>600
As OG explains, this is bigger than 600 but we do not know if it is less than 1000.
(2)
I write you what I did, I don't take the OG solution.
5^k-1 = 5^k-500
I just put 500 to the left and 5^(k-1) to the right consequently I change their sign.
500 = 5^k - 5^(k-1)
I factorize
500 = 5^(k-1)*(5 -1)
500 = 5^(k-1)*4
I divide both sides by 4
125=5^(k-1)
5²=25 5^3=125
So, 5^3=125 and we have 125=5^(k-1)
Then, 3=k-1, then k=4
Now, we can answer the question "Is 5^k less than 1000?"
5^4=625<1000
Hence, B is sufficient
-
- Legendary Member
- Posts: 661
- Joined: Tue Jul 08, 2008 12:58 pm
- Location: France
- Thanked: 48 times
When you multiply by a positive number --> Don't change anythingstep 1 - divide both sides of the given inequality by 5 (*see my note*)
5^k+1 ÷ 5 > 3000 ÷ 5 = 5^k > 600
** The book confuses me here I thought when you divide you're suppose to change the direction of inequality sign **
When you multiply by a negative number --> Change the direction of inequality
Read back what OG tells you, you may confuse something.step 2 - divide all terms by -1
(5^k ÷ -1) - (5^k-1 ÷ -1) = (-500 ÷ -1)
(5^k) - (5^k-1) = (500) **
** The book confuses me here - am I missing something? I thought when you divide all terms by -1, that 5^k will become - 5^k, that -5^k-1 ÷ -1 will be come +5^k-1