Solution:

Let the cost be C, Weight be W & k be constant

C∝$W^3$
Thus, C= k$W^3$

Firstly, we need to find k. Substitue the given value to find k
C=1000$
W= 27
1000=k$(27)^3$

You can use a calculator and save the value of k by using M+
$\frac{1000}{19683}$=k
k~0.050805

Now, we are asked to split the weight in the ratio 4:5 so that would be 12 decigrams and 15 decigrams
Substitute the above value of k and the weight to find cost.
C=0.50805*$(12)^3$
Cost of 12 decigram gold~$87.7914
Cost of 15 decigram gold=050805*$(15)^3$~171.46$
87.7914+171.46~259.26

Thus, $\frac{259.26-1000}{1000}$*100~-74%

IMO D

Hope this helps!