Lets be a price of the calculator - $ a then , after using the coupon, you need to pay $(a-18) and after using 15% discount , you need to pay (1-0.15)a=0.85a
then, if (a-18) will be more than 0.85a, you should prefer 0.15 % discount, because it will be cheaper,
a-18> 0.85a a-0.85a>18 0.15a > 18 a>120, that means that if the price of the calculator more than $120, 15% discount is better, but if the price of the calculator is less than $120, you should choose $ 18 coupon.
for example, we have the price of the calculator $100 100-18=82, 100*0.85 =85, coupon is better.
If the price of the calculator $200 200-18=182, 200*0.85=170, so 15% discount is better
if price of the calculator is $120, 120-18=102 120*0.85=102, it will not matter, what you are going to use, because you are going to pay the same amount of money
