According to common sense, it can't be superimposed, so let's just say it can't be superimposed.
The first question is that your algorithm is correct.
448+448/2=672>500
You can use a coupon of over 500 MINUS 35.
672-35=637
The second question means that you can get a 15% discount if you buy two pieces at one time.
448×2×0.85=76 1.6>500
Similarly, 76 1.6-35=726.6.
You are also right about the third question.
448+9.9=457.9>300
Only coupons over 300 minus 15 can be used.
457.9- 15=442.9