Discounted Price and Marked Price Calculation

Patel gets 20 shirts. For every 4 shirts, 3 are paid at marked price $x$ and 1 is paid at ₹ 100. So out of 20 shirts, there are 5 groups of 4 shirts. Number of shirts at marked price $= 5 \times 3 = 15$. Number of shirts at ₹ 100 $= 5 \times 1 = 5$.

Total amount $= 15x + 5(100) = 20000$ $15x + 500 = 20000 \implies 15x = 19500 \implies x = 1300$.