Rice is 20% costlier than wheat. By how much percent is wheat cheaper than rice?
Assume a bag of wheat costs =$1.00 per pound
Then a bag of rice would cost =$1.20 per pound
1.00 - [1.00/1.20] = 16 2/3 % - This is how much cheaper wheat is per pound. [Units don't matter, whether they are in pounds, kg, tons....etc.]
This means that if 1 pound of rice cost $1, then the cost of 1 pound of wheat would be: $1 - [0.1667 * $1] =0.8333 cents.