Present worth of a money when it is n years due

The present worth of a sum due sometime hence is ? 400 and the true discount is ? 80. Find the banker's gain.

The present worth of a sum due sometime hence is Rs 576 and the banker’s gain is Rs 16. The true discount is (a) Rs 36 (b) Rs 72 (c) Rs 48 (d) Rs 96

The present worth of bill due 7 months, hence is Rs. 1200. if the bill were due at the end of 2.5 years, its present worth would be Rs1016. Find the rate of interest of the bill.