Solutions; (1)c 15 method, 5c 10, 5c5,5 = 3003× 252 = 756756; (that is, five of 15 are multiplied by five of 10, and the last five are selected. The expression is the same. )

(2)C 12，4C8，4C4，4×A3，3=495×70×6=207900

(3)C 12，2C 10，5C5，5×A3， 1=66×252×3=49896