- the probability that a person has the virus given that they have tested positive is  0.0151.
- the probability that a person does not have the virus given that they have tested negative is  0.9999 
P(A) = 1/600 = 0.0017 
P(B) = 0.9 * 0.0017 + 0.1 * (1 - 0.0017) = 0.1014 
A) P (has the virus | tested positive) = P (tested positive | has the virus) ×   
                                                           P (has the virus)/ P (tested positive) 
                                                          = 0.9 × 0.0017/0.1014  
                                                          = 0.0151 
B) P (does not have the virus | tested negative) = P (tested negative | does not have the virus) × P (does not have the virus)/ P (tested negative) 
= (1 - 0.1) *× (1 - 0.0017)/ (1 - 0.1014)  
= 0.9999 
Probability is the department of mathematics regarding numerical descriptions of ways likely an occasion is to occur, or how possibly it's far that a proposition is genuine. The possibility of an occasion varies between zero and 1, wherein, roughly speaking, 0 suggests the impossibility of the occasion and 1 shows certainty. The better the possibility of an event, the more likely it is that the event will arise.  
A simple instance is the tossing of an honest (unbiased) coin. since the coin is truthful, the 2 results ("heads" and "tails") are both equally likely; the possibility of "heads" equals the chance of "tails"; and considering the fact that no different results are feasible, the possibility of both "heads" or "tails" is 1/2 (that could additionally be written as 0.5 or 50%). 
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This is a typical work problem in algebra. The approach to this is using the equation: Rt = 1, where R is the rate per person working, t is the amount of time worked by an individual. All their rates must equal to 1. The solution is as follows:
12/x + 15/x = 1
Solving for x,
x = 27 hours
So, if they work together, they can finish the work in 27 hours.
        
             
        
        
        
20/50=2/5=0.4=40%
So yeah, 2/5 or 0.40 or 40%
They're all equal.
Hope this helps.