question: How many more volunteers were there in 2015 compared to employees?
#1 return volunteers 
#2 return employees 
#3 return #1 in  2015 
#4 return #2 in  2015 
#5 return the  number of  #3 
#6 return the  number of  #4 
#7 return the  difference of #5 ,  #6