$4.$ If we consider that the company $($in question $3)$ manufacturers dishwashers in four stages and inspection occurs at each stage of the process. The average percentages of good$-$quality units are as follows.

Stage $1-87\%$

Stage $2-91\%$

Stage $3-94\%$

Stage $4-93\%$

a$.$ Determine the weekly product yield of good$-$quality dishwashers.

b$.$ what would weekly product input have to be acheive a final weekly product yield of $300$ dishwashers?