1. how to compute the relief u/s 89 (arrears of salary) for

Computation of relief in case of relief when salary has been received in arrears or in advance under section 89(1) (in terms of section 89 read with rule 21A):

1. Calculate the tax payable on the total income, including the additional salary of the relevant previous year in which the same is received. 
2. Calculate the tax payable on the total income, excluding the additional salary of the relevant previous year in which the same is received. 
3. Find out the difference in tax at (1) and (2) above. 
4. Compute the tax on the total income after excluding the additional salary in the previous year to which such salary relates.
5. Compute the tax on the total income after including the additional salary in the previous year to which such salary relates.
6. Find out the difference in tax at (4) and (5) above. 
7. The excess of tax computed at (3) over the tax computed at (6) is the amount of relief admissible u/s 89(1). No relief is however admissible if the tax computed at (3) is less than the tax computed at (6). In such a case the assessee–employee need not apply for relief.