Answer:
3 shirts
Explanation:
Each shirt costs originally $17.99 with a 20% discount. This means that you are paying 80% of the original price. The discounted price is $14.39.
$17.99 × 0.80 = $14.39
You need to buy enough shirts so that you spend at least $40. You need to round up since you can't buy a fraction of a shirt.
40 ≤ 14.39x
40/14.39 ≤ (14.39x)/(14.39)
2.779 ≤ x
2.779 ≈ 3
You will need to buy at least 3 shirts.