If Lina's father has a 15% off coupon, he will only have to pay 85% of the original cost. So the cost of the meal after the discount is:
Cost after discount = 85% of $20
Cost after discount = 0.85 x $20
Cost after discount = $17
Next, a 7% sales tax is applied to the cost after the discount:
Cost after tax = Cost after discount + (7% of Cost after discount)
Cost after tax = $17 + (0.07 x $17)
Cost after tax = $17 + $1.19
Cost after tax = $18.19
Therefore, Lina's father pays $18.19 for the meal after the discount and sales tax are applied.