How many years did it take for the amount of water boards to be reduced from about 3,500 to 25 in 2011?
return the  amount of water boards was  about 3,500 ;return the  amount of water boards was  25 ;return year of #1 ;return year of #2 ;return the  difference of #4 and  #3
['select', 'select', 'project', 'project', 'arithmetic']