Let XXX and YYY have the joint PDF f(x,y)=6(1−x−y)f(x, y) = 6(1-x-y)f(x,y)=6(1−x−y) for x,y≥0,x+y≤1x, y \ge 0, x+y \le 1x,y≥0,x+y≤1. What is E[X]E[X]E[X]?
1/4
1/3
1/2
2/3