两数字分别为a,b
EX=0.6a+0.4b=7/5
D(X)=E{[X-E(X)]^2}=E(X^2)-[E(X)]^2=0.6a^2+0.4b^2-49/25=6/25
解得,a=1,b=2
所以X1,X2分别为1,2