解答

P(甲>乙) = P(乙>甲)
P(甲>乙) + P(甲=乙) + P(甲<乙) = 1
→ 2P(甲>乙) + P(甲=乙) = 1

10~99 共有 90 個數
P(甲=乙) = 90/(90×90) = 1/90

→ P(甲>乙) = [ 1 - 1/90 ]/2 = 89/180

→ P(甲≥乙) = 1/90 + 89/180 = 91/180

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