Original practice · fully worked
Original variant: recover a two-tag overlap
An archive contains 300 records. An urgent tag appears on 180 records, an external tag appears on 150 records, and exactly 170 records carry one of these tags but not both. Calculate the number of records carrying both tags.
- A
30 records - B
80 records - C
150 records - D
170 records - E
250 records
Answer and calculation
If x records carry both tags, then the urgent-only and external-only counts are 180-x and 150-x. Their sum is 170, giving x=80 and choice B.
Setup
Setup
Let x denote the intersection of the urgent and external tag sets.
∣U∩E∣=x Model
Model
Express the exactly-one count as the sum of the two disjoint one-tag regions.
∣U∖E∣+∣E∖U∣=(180−x)+(150−x)=170 Compute
Compute
Solve the resulting linear equation for the overlap.
330−2x=170 2x=160,x=80 Answer
Answer
Exactly 80 records carry both tags.
80records (B)