Given details are,
Rate = 12 ½% per annum = 25/2% = 25/2/2 = 25/4% half yearly
Amount = Rs 4913
Time (t) = 18months = 18/12years = 3/2 × 2 = 3 half years
By using the formula,
A = P (1 + R/100) n
4913 = P (1 + 25/4 ×100)3
P = 4913 / (1 + 25/400)3
= 4913/1.19946
= 4096
∴ The principal amount is Rs 4096.