I have a 88.1% right now, if I get 15 out of 15 on this assignment what will my grade be

So you start with a 88.1%.

Here is how I figure it ...

If you get a 15 out of 15 on an assignment, that would be a 100% for your assignment grade.

You take 100% and add it to your 88.1% and you get 188.1%. That doesn't seem reasonable for a grade though does it ...

So you take that 188.1% and you divide it by 2 (divide it in half) and you get 94.05%.

So if you get 15 out of 15 on your assignment your grade will go up to a 94.05% as I figure.

Word problem:

15 out of 15 = 100%+88.1% = 188.1%:2 = 94.05%

Your final grade as I figure would be a 94.05%