Answer:
70
Step-by-step explanation:
(Not the most streamlined way but.)
Knowing that if he had 5 tests, and a mark of 85, we can determine the total average grade Bill received. 85*5=425
Then subtract the known grades
425-(90+85+100+80)=x
425-(355)=x
70=x
Bill would have scored 70 on that 5th test