Agricultural income is exempt under Section 10(1) of the Act so long as the income is derived from agricultural land situated in India. This income is, however, included merely for rate purposes and rebate is allowed on the same in accordance with the Finance Act.
Your tax calculation shall be as follows.

1. Add agriculture and business income(20000+30000).
2. Calculate tax on this income(5 lacs) as per the tax slab applicable to you.
3. Add basic exemption limit to the agriculture income you earned.(I assumed that you fall in a tax slab of 250000+200000)
4. Calculate tax on step 3
5. Deduct the result of step4 from step 2
6. Deduct rebate u/s 87A as your income does not exceed 5 lacs.
7. Add the education cess of 3% and the result would be your tax liability to pay.