having trouble finding out where 0.999 , 0.9999 & 0.9991 , 99.91, 99.91, 99.1/99.99 came from , please help

please dumb down explanation of how to solve problem please

4. As a design engineer you need to obtain 99.9% reliability. The first part has a reliability factor of 99.99%. What level of reliability do all the other parts combined require to achieve 99.9% overall reliability? Ans: 99.91%((R2R3Rn)=RS/R1=0.999/0.9999=0.9991=99.91%=99.9%/99.99% =99.91%) Solution: If all the parts in a product must work for the product to function, then the reliability of the system is computed as the product of the reliabilities of the individual components: Rss=R1R2R3.Rn;Rs=99.9%=.999;R1=99.99%=.9999 (R2R3Rn)=RsS/R1=0.999/0.9999=0.9991=99.91% Section Ref: The Philosophy of TQM Subsection Ref: Product Design