You said the 87% is an average. So, it does not matter whether we got that via let’s say, 5 scores of 87 each, or 85–86-87–88-89. Either way you have an 87 average.
Now, I assume that each assignment is equally weighted. So, take 87 and multiply it by the number of assignments (I’ll call this variable “n”) to get your total number of points (or just use your total if you know). Again, since 87 is an average, your actual scores do not matter.
Now just take however many total points you got and divide that by the number of assignments plus 1. This is your new score.
An e.g. with 5 assignments
Total score=5*87=435
Number of assignments=5
So, your new grade is 435 / (5+1) = 72.5