A project manager needs a certain number of identical valve housings for a power station project.
He starts with 100 raw items which are made as metal castings. Out of this batch, 15 are found leaking from micro-porosities in a subsequent pressure test. 6 of these housings can be recovered by a special sealing process for raw castings.

The 91 housings are then being machined. After this process step 10 machined castings are found leaking. 3 of these items can also be saved using another sealing process for machined castings.

The remaining 84 housings are getting mounted to the power plant pipe work. During a pressure test after this process step it is found that 12 valves are leaking. Out of these a number of 6 valves can be saved by replacing the sealing rings used.

Finally the project manager has 78 valves which he can use. What is true?

  RTY = 0.84
  RTY = 0.78
  RTY = 0.91
  RTY = 0.65
 

Ans: 0.65

If someone could please explain this.

Thanks

Nidhi