Purchased photo equipment at a cost of 105000 from camera works .she paid 1/3 and promised to pay the balance at the end of the month,
How much is the balance?

2/3.

A simple way to figure out your ammounts is to do a journal entry for the transaction:

Debit Photo Equipment for 105,000
Credit Cash for an amount that is equal to 105,000 * 1/3
Credit Accounts Payable for an amount that is equal to 105,000 * 2/3

Debits must equal credits, so (1/3 * 105,000) + (2/3 * 105,000) should equal 105,000