Figure 5.

Probability space of X ≥ k₁ ∧ Y ≥ k₂ is a subset of X + Y ≥ k₁ + k₂. P(X ≥ k₁ ∧ Y ≥ k₂) is the sum of the probabilities taken over all values in the dark gray space, while P(X + Y ≥ k₁ + k₂) is the sum of the probabilities taken over both the light gray and dark gray spaces.