In a bookstore, the average number of books in biology, chemistry, and physics is 360. The number of biology books is 180 more than the average number of chemistry and physics books. The number of physics books is 180 less than the average number of biology and chemistry books. How many chemistry books are there in the store?

Correct option is D

Given:
Average number of biology, chemistry and physics books = 360.
Biology books = 180 + average number of chemistry and physics books.
Physics books = Average of biology and chemistry books - 180
Solution:
Let the number of biology, chemistry and physics books be x, y and z.
$$\frac{x + y + z}3$$​ = 360
=> x + y + z = 1080 ----(i)
Biology books = 180 + average number of chemistry and physics books,
x = 180 + $$\frac{y + z}2$$​​
=> 2x = 360 + y + z
=> 2x - y - z = 360 ----(ii)
Physics = Average of biology and chemistry books - 180,
z = $$\frac{x + y}2$$​ - 180
=> 2z = x + y - 360
=> x + y - 2z = 360 ----(iii)
Adding equation (i) and (iii) and subtracting (iii),
(x + y + z) + (x + y - 2z) - (2x - y - z) = 1080 + 360 - 360
=> x + y + z + x + y - 2z - 2x + y + z = 1080
=> 3y = 1080
=> y = 360
∴ The number of chemistry books in the shop is 360.