Task: Question Answering
Input (English): Problem: a certain telephone company offers two plans , a and b . under plan a , the company charges a total of $ 0.60 for the first 4 minutes of each call and $ 0.06 per minute thereafter . under plan b , the company charges $ 0.08 per minute of each call . what is the duration of a call , in minutes , for which the company charges the same amount under plan a and under plan b ? 
Options: a ) 2 , b ) 9 , c ) 15 , d ) 18 , e ) 30
Output (English): d