Let x be the number of cans then the cost of x cans is 2.25x dollars.
The coupon is worth $3 so we can add this to the maximum $30 to make $33, which is the actual limit of her spending. Therefore 2.25x≤33 and x≤33/2.25. $2.25 can be written as $2¼ or $9/4. So x≤33×4/9, x≤11×4/3, x≤44/3, x≤14⅔. Since x must be a whole number, which is less than 14⅔, x must be 14 at most. So she can buy 14 cans of soup. 14×2.25=$31.50, but the coupon gives her a discount of $3 so she only has to pay $28.50. (Note that she can’t buy another can because she would go over budget by $0.75.)