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12 Jan 2012, 22:42
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In a certain sequence, every term after the first is determined by multiplying the previous term by an integer constant greater than 1. If the fifth term of the sequence is less than 1000, what is the maximum number of non-negative integer values possible for the first term?
A) 60
B) 61
C) 62
D) 63
E) 64
A) 60
B) 61
C) 62
D) 63
E) 64
In a certain sequence, every term after the first is determined by multiplying the previous term by an integer constant greater than 1. If the fifth term of the sequence is less than 1000, what is the maximum number of non-negative integer values possible for the first term?
A. 60
B. 61
C. 62
D. 63
E. 64
Given sequence:
\(x\);
\(x*r\);
\(x*r^2\);
\(x*r^3\);
\(x*r^4<1,000\) (where x is the first term and r is the constant greater than 1).
To maximize the # of non-negative integer values possible for \(x\), we should minimize the value of \(r\) and since \(r=integer>1\) then \(r=2\). (General rule for such kind of problems: to maximize one quantity, minimize the others and to minimize one quantity, maximize the others.)
Thus, \(x*2^4<1,000\) -->\(x<\frac{1,000}{16}=62,5\) --> as the first term must be a non-negative integer then: \(x_{max}=62\) and \(x_{min}=0\) --> total of 63 values possible for the first term x: {0, 1, 2, ..., 62}.
Answer: D.
A. 60
B. 61
C. 62
D. 63
E. 64
Given sequence:
\(x\);
\(x*r\);
\(x*r^2\);
\(x*r^3\);
\(x*r^4<1,000\) (where x is the first term and r is the constant greater than 1).
To maximize the # of non-negative integer values possible for \(x\), we should minimize the value of \(r\) and since \(r=integer>1\) then \(r=2\). (General rule for such kind of problems: to maximize one quantity, minimize the others and to minimize one quantity, maximize the others.)
Thus, \(x*2^4<1,000\) -->\(x<\frac{1,000}{16}=62,5\) --> as the first term must be a non-negative integer then: \(x_{max}=62\) and \(x_{min}=0\) --> total of 63 values possible for the first term x: {0, 1, 2, ..., 62}.
Answer: D.
enigma123
You can solve the problem using equations or take a logical route.
I'll do it by logical elimination.
We are given the following: T1, T2, T3..T5 are 5 terms of a sequence, where T2 = K*T1 , T3=K*T2 [K> 1 & T5 < 1000]
To get the MAXIMUM number of non-negative integer values for T1 , we have to take the value of K as low as possible and K> 1 (given) therefore let us take K=2. Looking at the options we can figure out that value of T1 will be b/w 60-64.
Lets us start by the maximum value of 64.
T1=64, T2=128......T5=1024 (but T5 < 1000), hence E is out
T1=63,T2=126........T5=1008 ,Hence D is out
T1=62, T2=64.........T5= 992. This works .Thus all values from 1 to 62 will work .
But we are told that T1 is a non-negative integer, so T1=0 (zero) will also work. Thus 0 to 62 values will work . Therefore total number of values equals 63 .Option D.
Alternatively equations can be formed as shown above T (n+1) = K* Tn and T5 <1000. substutuite for T5=1000 and get T1 (T1=62.5) ,but T1 has to be integer therefore T1=62 ,then the method as above. Hope this helps
What is the source of the problem and the answer?
