Given: x and y are real numbers. I. Given: x+y=16 There are two unknowns and only one equation. Further, x and y being real numbers, we can have infinite number of solutions. ∴ Ι alone is not sufficient II. Given: x2+y2=160 Similarly, II alone is not sufficient. I&II. x+y=16,x2+y2=160 (x+y)2=x2+y2+2xy. So we can find xy. ∴ I&II together are sufficient.