Pre-tax return on net worth= Pre tax income/net worth of the investment
However, in reality investors has to pay taxes on the earnings that they get from an investment. So, after-tax return is more important for the investors.
An investment that gives 20% return on investment would give 120 rupees for every 100 rupees invested. But if a tax of 5% is charged on the earnings. The investor would effectively get 119 on every 100 rupees invested. So after tax rate of return would be 19%.