findingmyself
Consider the equation \(\frac{1}{x}\)+\(\frac{1}{y}\)+\(\frac{1}{z}\)=\(\frac{4}{3}\), where x, y and z are positive integers. Which of the following could be the value of x+y+z?
I.5
II.7
III.13
A) I only
B) II only
C) III only
D) I and II only
E) II and III only
\(\frac{1}{x}\)+\(\frac{1}{y}\)+\(\frac{1}{z}\)=\(\frac{4}{3}\)
\(\frac{yz + xz + xy}{x*y*z} = \frac{4}{3}\)
Observation1. x * y * z is a multiple of 3. Therefore, one of the three numbers must be a multiple of three.
2. yz + xz + xy is a multiple of 4, hence the value is even. This is possible when all three numbers are even, or when two numbers are even and one is odd.
Let's look at the available options
1. x + y + z = 5Assume x = 3. Therefore y = 1 and z = 1. Note, it doesn't matter which of the variable is assigned the value 3 as the remaining sum (y + z), will be adjusted between the remaining two variables.
In this case, all the three numbers are odd. Hence, the sum yz + xz + xy cannot be even.
Hence, we can conclude that \(x + y + z \neq 5\)
We can eliminate A and D.
2. x + y + z = 7As x * y * z must be a multiple of 3, let's assume x = 3
Possible values of x, y and z =
- x = 3, y = 1 , z = 3 → Ignore, as all three values are odd.
- x = 3, y = 2, z = 2 → Let's calculate the sum of the reciprocals
\(\frac{1}{x} + \frac{1}{y} + \frac{1}{z} = \frac{1}{3} + \frac{1}{2} + \frac{1}{2} = 1 + \frac{1}{3} = \frac{4}{3} \)
The value matches, x + y + z can be 7.
The possible answer is between B and E. Eliminate C.
2. x + y + z = 13Possible values of x, y and z =
- x = 3, y = 1 , z = 9 → Ignore as all three values are odd
- x = 3, y = 2, z = 8 → 1/3 = 0.33 ; 1/2 = 0.5 ; 1/8 = 0.125 ⇒ Sum is less than 1.33. Hence, we can ignore this combination.
- x = 3, y = 3, z = 7 → Ignore as all three values are odd
- x = 3, y = 4, z = 6 → 1/3 = 0.33 ; 1/4 = 0.25 ; 1/6 = 0.167 ⇒ Sum is less than 1.33. Hence, we can ignore this combination.
- x = 3, y = 5, z = 5 → Ignore as all three values are odd
- x = 6, y = 1, z = 6 → 1/6 = 0.167 ; y = 1/1 = 1 ; z = 1/6 = 0.167 ⇒ Sum = 1 + 0.33 = 1.33. Matches & we don't have to try out more combinations.
Hence, x + y + z can be 13.
Option E