Frank and Brenda work at a bank. They’re the only ones who can get into the bank vault. Julie just started working there and hasn’t used the vault lock by herself yet.
The bank vault has a complex lock. It needs 5 numbers between 0 and 100 in a specific order that’s hard to guess. You have to spin the knob the right number of times for each of the 5 numbers in order to open it.
However, the lock operation has been kept simple to help employees remember the number of times they need to turn the combination knob before entering the actual number. If someone turns the knob the wrong number of times at any point, the lock resets. After 3 resets, the lock blocks access for the next 24 hours—even if you enter the right combinations.
During Christmas, the bank called Brenda to help the police with what seemed to be a robbery. Frank was on vacation out of town and not answering his phone.
When Brenda got to the bank, they asked her to open the vault to check if everything was okay. She knew the vault code so she entered it :
- First number: 36 — input follows 3 turns
- Second number: 72 — input follows 5 turns
- Third number: 89 — input follows 1 turn
- Fourth number: 84 — input follows 4 turns
- Fifth number: 92 — but Brenda can’t remember the number of turns needed!
She gave it her best shot but ended up resetting the vault lock. Another reset means the vault stays closed for a full day.
The Task
Can you work out how many turns Brenda needs to make before she inputs the final number (92) to get into the vault?
—– View the answer —–
Brenda has to turn the knob seven times and then input 92 to unlock the vault !
The method is pretty easy for employees to keep in mind. Subtract one digit of the code number from the other to figure it out i.e. 9 – 2 = 7.
