For any positive x, y and z if x is n% of y and z is m% of y then what percentage must x be of z?
A. (n/m)%
B. (m × n)%
C. (100 / [m × n]) %
D. (100 × m/n)%
E. (100 × n/m)%

(E) The statement x is n% of y means that:
x = (n / 100) × y

The statement z is m% of y means that:
z = (m / 100) × y

In order to find what percentage is x of z we need to divide x by z.
x / z =

(n / 100) × y=
(m / 100) × y

= n / m

Therefore x = (n / m) × z
x = [(100 × n / m) / 100] × z

or
x is (100 × n / m)% of z.

The correct answer is choice (E).
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Hi, there is some ambiguity in your answers. Here I chose option (A), but you gave answer as (E). In Math4 Test Q.no.27, which is similar to this question, you have given answer as (A), though I chose answer (E). Can you explain this?

These are two similar but DIFFERENT questions. The have different answers. There is no ambiguity in that. Different questions have different answers.

The similar question: http://www.gmat-mba-prep.com/forum/gmat-percentage-t24906.html

Now let's discuss the things these questions have in common.
1. They deal with percentages.
2. No specific integers are given, just variables.

We use the same plan in both questions.
1. Transform wording into algebra.
The statement x is n% of y means that:
x = (n / 100) × y
The statement z is m% of y means that:
z = (m / 100) × y

2. Transform the original formula into x = (P / 100) × z, which the same as wording x is P% of z.
x / z = n / m
x = (n / m) × z
x = [(100 × n / m) / 100] × z

P% is the answer.
E. (100 × n / m)%