Question

19 Zul's password to unlock his mobile phone screen consists of 4 digits. Each digit can be chosen from 0, 1, 2, ..., 9. What is the probability to unlock the screen in one attempt if he knows (a) the first three digits are 970? (b) there are three 1s? (c) the last two digits are 00?​

Answer 1

(a) Since Zul's password consists of 4 digits and the first three digits are 970, there is only one possible digit for the fourth digit. Thus, the probability to unlock the screen in one attempt is 1/10.

(b) If there are three 1s in the password, there is only one possible arrangement for the other digit. There are four places where the other digit can be placed, so there are four possible passwords. Thus, the probability to unlock the screen in one attempt is 4/10^4, which simplifies to 1/2500.

(c) If the last two digits are 00, there is only one possible arrangement for the first two digits. There are ten possible digits for the third digit and one possible digit for the fourth digit. Thus, the probability to unlock the screen in one attempt is 1/(101010*1), which simplifies to 1/1000.