The average of 4 terms is 20 and the 1st term is 1/3 of the remaining terms. What will be the first number?

Question: The average of 4 terms is 20 and the 1st term is 1/3 of the remaining terms. What will be the first number?

Solution:
Average of 4 terms = 20
Hence, the total sum of 4 terms = 80
Let terms be A, B, C & D

So,
The sum will be A + B + C + D =80
Given, 3A = B + C + D
So, 4A = 80,
A = 20