Steps
So remember that the average formula is [tex]\frac{\textsf{sum of all the numbers}}{\textsf{how many numbers there are}}[/tex] . With the info that we are given, our equation is (Let x = fourth number):
[tex]\frac{138+40+102+x}{4}=90[/tex]
From here we can solve for x. Firstly, add up the numerator:
[tex]\frac{280+x}{4}=90[/tex]
Next, multiply both sides by 4:
[tex]280+x=360[/tex]
Lastly, subtract both sides by 280:
[tex]x=80[/tex]
Answer
In short, the fourth number is 80.
