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?

999 unsuccessful attempts can be made...there are totally 10 letters possible for each ring and there are 3 such rings...therefore totl no. of possible code is 10*10*10=1000;

out of 1000,only 1 code is correct...therefore 999 unsuccessful attempts are possible...