The number of books on the two shelves was the same. When 18 books were transferred from the first shelf to the second

The number of books on the two shelves was the same. When 18 books were transferred from the first shelf to the second, the number of books on the second shelf was 3 times more than on the first. How many books are on each shelf originally.

Suppose there were originally x (x) books on two shelves. After putting 18 books on the first shelf, there were x – 18 books, and on the second: x + 18 books. Knowing from the condition that now there are three times as many books on the second shelf, we will compose the equation:
(x + 18): (x – 18) = 3
x + 18 = (x – 18) • 3
x + 18 = x • 3 – 54
x – x • 3 = – 54 – 18
– x • 2 = – 72
x = 72: 2
x = 36
Answer: initially there were 36 books on each shelf.