其它
Find the next number in the following sequence  $$21, 24, 19, 26, 17, \cdots $$ 
Overseas Competition->Knowledge Point->Calculation Modules->Sequences and Number Tables->Patterns in Number Sequences
$$21, 19, 17, \cdots $$ (pattern of $$-2$$)  $$24, 26, ?, \cdots $$ (pattern of $$+2$$)  Thus, the "?" is equal to $$26+2=28$$ 