Answer:
Let x represent the amount that the supermarket paid for the box. We were told that the mark up was 20%. This mean that the amount, x was increased by 20%.
Recall, percentage is expressed in term of 100.
This mean that the amount by which it was increased is
20/100 * x = 0.2x
The selling price would be
x + 0.2x = 1.2x
If the supermarket sells the corn flakes for $3.6 per box, it means that
1.2x = 3.6
x = 3.6/1.2
x = 3
Thus, the amount that the supermarket paid for each box is $3
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