In bank reconciliation statement,
Bank balance as per cash book 6000,a cheque drawn 1800 entered in cash book as 800,find bank balance as per passbook?

You need to reduce your cash balance per your books by $1,000 because you entered the check in your books for $800, but it was posted as cashed by the bank for $1,800

When you do a bank reconciliation you need to get the balance per your books and the bank to equal.

So you need to reduce your cash balance by the amount calculated above, which should equal your bank balance, unless of course there are other items that you did not list.