Under a simplified scheme the first ₹250000 of a year's income is tax-free, the next ₹250000 is taxed at 5% and everything above ₹500000 at 20%. Harpreet's tax for the year comes to ₹42500. Her annual income (in ₹) is:

The middle slab alone yields 5% of ₹250000 = ₹12500, so the top slab must yield 42500 − 12500 = ₹30000, i.e. 20% of ₹150000. Her income is 500000 + 150000 = ₹650000. Forgetting the ₹12500 already paid gives 500000 + 42500/0.20 = ₹712500, while 42500/0.05 = ₹850000 and 42500/0.20 = ₹212500 tax the whole income at one slab rate.