Correct Answer: 90

Solution : Given: There are two numbers such that the first number is 5 times the second number.
The difference between the first and the second numbers is 120.
Let the first number be $x$.
Then, the second number = $5x$.
According to the question,
$5x-x=120$
⇒ $4x=120$
⇒ $x=30$
The numbers are $x=30$ and $5x=5\times 30=150$
The average of these two numbers = $\frac{30+150}{2}=\frac{180}{2}=90$
Hence, the correct answer is 90.