Given,
Rate = 15% p.a
Time = 2 years
C.I = Rs. 1290
Let principal = P
So,
Compound interest \(={P}[({1}+\frac{R}{100})^T-{1}]\)
\(={P}[({1}+\frac{15}{100})^2-{1}]\) = 1290
\(={P}[(\frac{23}{20})^2-{1}]\) = 1290
\(={P}\times\frac{129}{400}\) = 1290
\(={P}=\frac{1290\times400}{129}\) = Rs. 4000
Hence,
Principal = Rs. 4000