Four times a number added to 9 times the number is 65. What is the number?
2 answers:
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<h3>Answer: 0.4545</h3>
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Work Shown:
Define the following events
- B = the man takes the bus
- S = the man takes the subway
- L = the man is late
- L' = the man is not late (ie the man is on time)
Draw out a probability tree. See diagram below.
From the tree diagram, we can figure out that P(L) = 0.44 which is the probability of being late. Note how I considered the two probabilities P(B & L) and P(S & L)
- P(B & L) = probability of taking the bus and being late
- P(S & L) = probability of taking the subway and being late
The events (B & L) and (S & L) are mutually exclusive, so we can add them up. Combining them will have us find the overall probability of being late. For more information, check out the law of total probability.
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Now use a conditional probability formula to find...
P(B|L) = probability the man takes bus given he is late
P(B|L) = P(B & L)/P(L)
P(B|L) = 0.2/0.44
P(B|L) = 0.45454545454546 approximately
P(B|L) = 0.4545 rounding to four decimal places
note: when I write "P(B|L)", I am using a vertical bar. I'm not using an uppercase letter 'i' or a lowercase letter L.