<p>5 * m^5 * n^-3 = 20 * m^3 * n</p>
<p>Let n be 2</p>
<p>5 * m^5 * 2^-3 = 20 * m^3 * 2
5 * m^5 * .125 = 40 * m^3
.625 * m^5 = 40 * m^3
m^5 = 64 * m^3
m^2 = 64
m = 8 (m=-2 won’t do since m>0)</p>
<p>The right answer would be one that turns out to be 8 when we plug n=2 into the corresponding expression.
(A) 1/4n
(B) 4/n^2
(C) 4/n^3
(D) 2n^2
(E) 4n^2</p>
<p>Here goes nothing. 
We can quickly rule out A. B. and C as too small.
D produces 2*2^2=8 - that works; E will be greater than D.
That leaves D only.</p>