A Boy read 3/8 of the book on one day and 4/5 of the remainder on the other day if there were 30 pages unread,how many pages did the book contain?

Let the total no. of pages be 'b'

Fraction of book read on Day 1 - 3b/8

Remainder - b - (3b/8) = 5b/8

Fraction of book read on Day 2 - (5b/8)(4b/5) = (20b/40) = b/2

Remainder = b - { (3b/8) + (b/2) } = b - { (3b + 4b)/8 } = b - (7b/8) = b/8

Given b/8 = 30 pages 

Therefore, b = 30 * 8 = 240 pages

nice answer @niyor