if there are 5 options and negative marking of less than 0.25, then its is better to guess than to leave it, on average. 
similarly, if there are 4 options and negative marking of less than 
1/
3, then it is better to guess than to leave it, on average.
 
 
got stat tmw 
 
 
(and its the only thing i learnt in accg253)