Question

Susan has 3 lists, each with 10 numbers. If there are 4 numbers on all three lists and 5 numbers on exactly 2 lists, how many numbers belong to just one list?

Answer 1

Susan has 8 numbers belonging to just one list.

Explanation:

Susan's 3 lists have 10 numbers each = 10 x 3 = 30 numbers

4 numbers appear on all three lists = 4 x 3 = 12 numbers

The remaining numbers after these 12 = 18 (30 -12)

Then, there are 5 numbers on 2 lists only = 5 x 2 = 10 numbers

The numbers on just one list = 18 - 10 = 8 numbers

Or

List 1 List 2 List 3 Total

Numbers on each list 10 10 10 30

Numbers on 3 lists -4 -4 -4 12

Numbers on 2 lists -5 -5 -0 10

Numbers on 1 list only 1 1 6 8