Question

A letter lock contains 3 rings, each ring containing 5 different letters. Determine the maximum number of false trials that can be made before the lock is opened.

Answer 1

$\overline{5}\times \overline{5}\times \overline{5}$

Each ring can be set in 5 different

ways

Hence total ways $=5\times 5\times 5=125$

unsuccessful attempt = 125 - 1

= 124

[As one combination would be the correct one]