Task: Question Answering
Input (English): Problem: if x + y = 2 x + z , x - 2 y = 4 z and x + y + z = 21 , what is the value of y / z ? 
Options: a ) - 4.5 . , b ) - 2 . , c ) - 1.7 . , d ) 3 . , e ) 6 .
Output (English): e