Task: Question Answering
Input (English): Problem: at the end of the month , a certain ocean desalination plant ’ s reservoir contained 30 million gallons of water . this amount is twice the normal level . if this amount represents 75 % of the reservoir ’ s total capacity , how many million gallons short of total capacity is the normal level ? 
Options: a ) 10 , b ) 15 , c ) 20 , d ) 25 , e ) 30
Output (English): c