Answer:
4
Step-by-step explanation:
If all packages contained 5 markers, then 35 markers would have been sold. Instead, a number 12 fewer than that was sold.
The sale of a 2-marker package instead of a 5-marker package reduces the marker count by 3 for each 2-marker package. Hence there were 12/3 = 4 packages of 2 markers sold.
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If you like, you can write an equation for x = the number of 2-marker packages. Then the total number of markers sold was ...
2x +5(7-x) = 23
-3x +35 = 23 . . . . simplify
-3x = -12 . . . . . . . . subtract 35
x = -12/-3 = 4 . . . . divide by 3
4 packages contained 2 markers.
