Hi

x/y = 1/3 ----> 3x=y

z/y = 2/5 ----> 5z = 2y

5z = 6x -----> z=6n, x=5n

z + x = 6n + 5n = n(6 + 5) = 11n

z + x should be a multiple of 11.

Answer E (66)

Given: x,y, z>0
x/y=1/3
z/y=2/5

As y is common in both the ratios, I equated them.
y=3x and y=5z/2
3x=5z/2
x=5z/6

That means: z is divisible by both 2 and 3 (x is an integer). Therefore z contains at least 2 prime factors 2 and 3 as 5 is not divisible by 6.

x+z=5z/6 + z= 11z/6
z is divisible by 6 (2 and 3) and the answer is a multiple of 11.
Therefore the answer is 66 (E)
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Given: x : y = 1 : 3
y : z = 5 : 2

To COMBINE these ratios, their shared term (the y-value) must be the same in each ratio.

So, take x : y = 1 : 3 and multiply both parts by 5 to get an EQUIVALENT RATIO: x : y = 5 : 15
Likewise, take y : z = 5 : 2 and multiply both parts by 3 to get: y : z = 15 : 6
We can now COMBINE the ratios to get: x : y : z = 5 : 15 : 6
This means the ratio x : z = 5 : 6

Since x and z are positive integers, x + z must be a MULTIPLE of 5 + 6
In other words, x + z must be a MULTIPLE of 11

Answer: E

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Brent Hanneson – Founder of gmatprepnow.com