Question

The key of a lock is in a bunch of 10 keys. Keys are drawn

one by one at random and tried to open the lock. The wrong

key is discarded after trial. Then the probability that the lock

is opened in 6th attempt is

• A. 1/10
• B. 1/5
• C. 9/10
• D. 4/5

Answer 1

Answer: (A)

1/10

Answer 2

P(5) = $\frac { 9 } { 10 } \times \frac { 8 } { 9 } \times \frac { 7 } { 8 } \times \frac { 6 } { 7 } \times \frac { 5 } { 6 } \times \frac { 1 } { 5 } = \frac { 1 } { 10 }$.