Question

A and B throw a die alternatively. If A starts throwing

then the probability that he throws 5 before B throws 3 or 6 is

• A. 3/8
• B. 1/8
• C. 5/8
• D. 7/8

Answer 1

Answer: (A)

3/8

Answer 2

P(5) = P₁ + P₃ + P₅ + ............

= $\frac { 1 } { 6 } + \frac { 5 } { 6 } \times \frac { 2 } { 3 } \times \frac { 1 } { 6 } + \left( \frac { 5 } { 6 } \times \frac { 2 } { 3 } \right) ^ { 2 } \times \frac { 1 } { 6 } + \ldots \ldots$

= $\frac { \frac { 1 } { 6 } } { 1 - \frac { 10 } { 18 } } = \frac { 1 } { 6 } \times \frac { 18 } { 8 } = \frac { 3 } { 8 }$ .