prob. of a system can be a failure is less than (1-0.9999) = 0.0001
now probability that the system will be failure when it has x components is
(0.2)^x
so, (0.2)^x < 0.0001
or, x*log0.2 < log0.0001
or, x > log0.0001/log0.2 [the sign changes coz log 0.2 is negative]
or, x > 5.7227
so, the no. of components (x) has to be 6
