Answer:
0.625 = 62.5% probability that part B works for one year, given that part A works for one year.
Step-by-step explanation:
We use the conditional probability formula to solve this question. It is

In which
P(B|A) is the probability of event B happening, given that A happened.
is the probability of both A and B happening.
P(A) is the probability of A happening.
The probability that part A works for one year is 0.8 and the probability that part B works for one year is 0.6.
This means that 
The probability that at least one part works for one year is 0.9.
This means that: 
We also have that:

So


Calculate the probability that part B works for one year, given that part A works for one year.

0.625 = 62.5% probability that part B works for one year, given that part A works for one year.
5 1/7 x 1/9= 36/7 ×1/9= 4/7
8-3/2= 8-1 1/2= 7 1/2
This is only one equation. It is therefore not a system and you can not solve for two different variables y and z unless you have another equation to solve with. It.