Software Reliability is the probability of failure-free software operation for a specified period of time in a specified environment.
90 % of the code has probability of success as .9.
Let we have 2 blocks one is of 90% of unit and other is of 10%. Let first and second block both has .9 reliability then total reliability is .9*.9=.81.
Since 2nd block will have reliability always less than .9 (given) hence total reliability will be (.9*(no less than .9) hence it will be atmost .81.