Another way that you could approach it is through the answer choice, making the process quicker.
Looking at answer choice B. We can see that we have 1:5-->this telling us that we have 1 unit of A and 5 units of B all together having 6 units of rice.
Following upon this we are able to determine that we have 35C in the aforementioned mixture in costs.
We then multiply 7*6-->since we know what amount of units of rice we have and we know what the selling price is. We get 42.
We divide 42/35= 1,2-1=0,2*100=20%
Thereby answer choice B is correct.
Hope this helps