下面一串数是一个等差数列:2,5,8,11,…,212.这串数的平均数是 .
( )-258=159 256+( )=589
( )+356=912 128-( )=11
( )+125=352 408-( )=205
( )+351=652 125-( )=22
( )+325=561 641-( )=365 ( )-158=369 356+( )=515