Let the price of Wheat =X per kg. Then, the price of Rice =5X per kg (As it is given that the price of Wheat is $20\%$ the price of Rice.)
 Wheat                                           Rice
  250                                               90
$\frac{x}{250x}$                                             $\frac{5x}{450x}$
700 X = 3500
X=5
Hence the price of wheat is = Rs.5 per kg
Let the new amount of wheat be M kg,then $M\times 5+90\times 30=3500;M=160$
(As the price of Rice increases by $20\%$  i.e., from Rs.25 to Rs.30)
Hence decrease (in $\%$) of amount of wheat $=\frac{250-160}{250}\times 100=36\%$
Alternate Method:
As wheat is $20\%$ of rice, instead of consuming 250 kg of wheat, Aman can consume 50 kg of rice. Hence,
$(50+90)x=3500\Rightarrow X=25.$
Now,x(25)+90(30)=3500.$\Rightarrow$ x=32.
Therefore,change in consumption =50 - 32=18kg.
Thus,percentage increase will be $\frac{18}{50}$  or $36\%$