A conditional probability question limits who can be picked, so the table’s grand total may not be the denominator. Restrict the group first, then count the target inside it. For “exactly one” activity, add the two “only” categories, not “both.” Recover a missing cell from its column total.
Hints
- Hint 1
The phrase “from the visitors who participated in at least one” limits who can be picked. That group includes everyone except visitors in the Neither activity column. Which totals give you its size?
- Hint 2
A column total counts both teens and adults. To find the missing Teen count under Author event only, subtract the Adult count from that column’s total.
- Hint 3
The two “only” columns hold visitors who did exactly one activity. Add their Teen counts for the numerator. The Both column holds visitors who did two activities.
Step-by-step
Restrict the group, then count the teens
Step 1Find the group the visitor is picked from
The phrase “at least one” is a condition: it limits who can be picked. The table shows visitors in all and who did neither activity, so type . Desmos shows . That’s the denominator because only those visitors could be selected.
- Step 2
Fill the missing Teen count
The Author event only column has visitors in total, including adults. Type ; Desmos shows . So teens attended only the author event.
- Step 3
Count teens who did exactly one activity
The Teen row has visitors in Reading challenge only and in Author event only. Exactly one means either of these categories, so type . Desmos shows . The Both cell also contains , but those visitors did two activities and don’t belong in this count.
- Step 4
Find the probability within the restricted group
Put the matching teens over the visitors who could be selected. Type ; Desmos shows . Tap the fraction button on that line to see . So the probability of selecting a teen who did exactly one activity is . Grid in 12/31.