Task: Question Answering
Input (English): Problem: there has been successive increases of 15 % and then 10 % in the price of gas from the previous month . by what percentage should a driver reduce gas consumption so that the expenditure does not change ? 
Options: a ) 12 % , b ) 15 % , c ) 18 % , d ) 21 % , e ) 24 %
Output (English): d