c( 10,4)*2^4=2 10* 16=3360

(2) It is equivalent to selecting two pairs from 10 pairs.

C( 10，2)= 10*9/2=45

(3) Choose one pair first, then two pairs, one for each pair.

C( 10， 1)*C(9，2)*2？ = 10*36*4= 1440