Provison for write down in inventories

In an accounting text manual, I note that the amount of inventories recognised as an expenses is as follows: In income statement Cost of opening inventory( $o) + purchase ( $890,000) - closing inventory before provisions for write down ( $140,000) - increase in NRV provision ($30,800) = $719,200 In the B/S : The closing inventory is $109,200 ($140,000-$30,800) Disclosure: write down of inventories: $30,800 Would the tutor mind advising what are the accounting entries involved in recording the provisions for the write-down of inventories ( why is the provision subtracted from the cost of sales)?