Ravi,

Read carefully what Erik suggest thousand times before : 80C51 Family Architecture Guide from Philips site.

Just for you, here is a small extract:

The DA A instruction is for BCD arithmetic operations. In BCD
arithmetic, ADD and ADDC instructions should always be followed
by a DA A operation, to ensure that the result is also in BCD. Note
that DA A will not convert a binary number to BCD. The DA A
operation produces a meaningful result only as the second step in
the addition of two BCD bytes.

Silviu