A letter lock consists of three rings each marked with 10 different letters. In how many ways it is possible to make an unsuccessful attempt to open the lock?

A letter look consists of three rings marked with 15 different letters. If N denotes the number of ways in which it is possibel to make unsuccessful attempts to open lock, then

A number lock on a suitcase has three wheels each labelled with ten digits 0 to 9. If opening of the lock is a particular sequence of three digits with no repeats, how many such sequences will be possible? Also, find the number of unsuccessful attempts to open the lock.

There are three copies each of 4 different books. In how many ways can they be arranged in a shelf?

In how many ways can 4 letters be posted in 3 letter boxes?

In how many ways can 4 letters be posted in 5 letter boxes?

In how many ways can 7 letters be posted in 4 letter boxes?

In how many ways can 5 letters be posted in 4 letter boxes?

In how many ways can 4 letters be posted in 5 letter boxes?

A code consists of two letters of the alphabet followed by 5 digits .How many such codes are possible ?

In how many ways the letters of the word RAINBOW be arranged ?