Answer:
90
Step-by-step explanation:
a)
Since, to find the average of a set of numbers, you need to add them all together and divide by the amount of numbers:
[tex]\dfrac{86+83+81+x}{4}=85[/tex]
b)
Multiply both sides by 4:
[tex]86+83+81+x=340\\x=90[/tex]
Hope this helps!
