j_shreyans wrote:If x is a positive integer, what is the value of x?
(1) The first nonzero digit in the decimal expansion of 1/x! is in the hundredths place.
(2) The first nonzero digit in the decimal expansion of 1/(x+1)! is in the thousandths place
Mitch's solution is great.
We can also solve the question without finding an approximate value of each term. Here's how:
First make the following observations:
1/1 = 1
1/10 = 0.1
1/100 = 0.01
1/1000 = 0.001
So, we get:
1/1 = 1
1/2! = 1/2 = 0.5
1/3! = 1/6 = 0.something [if 1/6 is BETWEEN 1/10 and 1/1, then 1/6 is between 0.1 and 1, which means 1/6 = 0.something]
1/10 = 0.1
1/4! = 1/24 [if 1/24 is BETWEEN 1/100 and 1/10, then 1/24 is between 0.01 and 0.1, which means 1/24 = 0.0something]
1/100 = 0.01
1/5! = 1/120 = 0.00something.
1/6! = 1/720 = 0.00something.
1/1000 = 0.001
Okay, onto the question.....
Target question: What is the value of x?
Statement 1: The first nonzero digit in the decimal expansion of 1/x! is in the hundredths place.
As we can see above, ONLY 1 value of x (x = 4) satisfies this condition.
So,
x MUST EQUAL 4
Since we can answer the
target question with certainty, statement 1 is SUFFICIENT
Statement 2: The first nonzero digit in the decimal expansion of 1/(x+1)! is in the thousandths place
As we can see above, 1/5! and 1/6! both satisfy this condition.
This means that EITHER (x+1) = 5 OR (x+1) = 6
In other words, EITHER
x = 4 OR
x = 5
Since we cannot answer the
target question with certainty, statement 2 is NOT SUFFICIENT
Answer =
A
Cheers,
Brent
