Income tax calculation in AY 2019-20 applies to annual earnings in FY 2018-19. For the applicable AY, basic exemption limit is Rs. 2.5 lakh, and the slab rate is 5% for income of Rs. 250,001 to Rs. 5 lakh. Income above Rs. 5 lakh will have a 20% tax rate as per IT slabs for AY 2019-20. So you have a tax liability of 20% on Rs. 20,000 = Rs. 4,000. Additionally, 5% tax on 2.5 lakh (Rs. 5 lakh – Rs. 2.5 lakh) = Rs. 12,500. Thus your total tax liability for AY 2019-20 is Rs. 16,500. You can of course get the same result using a free online income tax calculator.