Answer:
100
Step-by-step explanation:
Given Average After writing a last test=90
let the last test score be x
Now given
[tex] \frac{80 + 85 + 100 + 85 + x}{5} = 90[/tex]
[tex]80 + 85 + 100 + 85 + x = 450[/tex]
[tex]x + 350 = 450[/tex]
[tex]x = 100[/tex]
I hope it helped you