World Electronic produces computerized sewing machines. The sewing machine is made of 3 subsystems namely  ,   and  . The sewing machine will operate successfully if all the subsystems work.
• Subsystem   works if Transistor   or   works.
• Subsystem   works if Module   and Module   works.
• Subsystem   works if Chip 1 and 2 work or Chip 3 and 4 work.
The success rate of the Transistor  ,  , and Module  ,   is 0.85. The success rate of the Chip 1, 2, 3 and 4 is 0.8.
(a) Sketch a block diagram and calculate the probability a computerized sewing machine operates successfully.
(b) The management of World electronics has decided to improve the success rate of the computerized sewing machine by 10%. Decision was made to replace Chip 1, 2, 3 and 4 with Chip   which has a higher success rate. Same Transistor  ,   and Module  ,   to be used. As the Design Engineer of the sewing machine, suggest the minimum success rate that Chip   must have to achieve the target.