Answer: 36%
The diagrams are below.
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Step-by-step explanation:
There isn't a real need to use a diagram. We simply multiply the decimal values of each percentage given
- 90% ---> 0.90
- 40% ---> 0.40
There is a 0.90*0.40 = 0.36 = 36% chance of the instructor seeing Tess hit the bullseye.
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More formally,
A = Tess hits the bullseye
B = The instructor watches
P(A) = 0.90
P(B) = 0.40
P(A and B) = P(A)*P(B) since the events are independent
P(A and B) = 0.90*0.40
P(A and B) = 0.36
P(A and B) = 36%
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The diagrams are shown below if you still need them.
Figure 1 shows the probability tree. The values in red are the probability for each branch. The values in blue are the result of multiplying the red values along a certain pathway.
Example: Follow the left-most pathway to see 0.90*0.40 = 0.36 is the probability of "hits bullseye" and "instructor is watching" happening at the same time.
Figure 2 shows another way to view probability. We start with a 1 by 1 square. Its area is 1*1 = 1 square unit. This square is sub-divided into 4 regions marked A, B, C, D
- A = hits bullseye, instructor watches
- B = hits bullseye, instructor is not watching
- C = misses bullseye, instructor watches
- D = misses bullseye, instructor is not watching
Take note how region A is 0.90 units tall and 0.40 units across. Its area is 0.90*0.40 = 0.36; so this is a visual tool to see why multiplication takes place for problems like this. Regions C through D are a similar story.
We can arrange the results into a table like so

Notice that all four values add to 1.