@Vaishali.
If B comes at 9, A can come at any time after 9
If B comes at 9.01, A can come at any time after 9.01
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If B comes at 9.59, A can come only at 10.
So, favourable events = 59+ 58+........+2+1
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lets consider sample space.
If A arrives at 8, B can arrive anytime between 9-11 i.e. 120 min
If A arrives at 8.01, B can arrive anytime between 9-11 i.e. 120 min
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If A arrives at 10.00, B can arrive anytime between 9-11 i.e. 120 min
THus, sample space = 120 * 120
So, probablity = 1/8 (approx)