Radha borrowed Rs 12000 from her friends. Out of that, Rs 4000 were borrowed at 18% and the remaining at 15% rate of interest per annum. What is the total interest after 3 years?

Total amount borrowed by Radha = Rs 12,000
We know that, simple interest  = $\frac{{\mathrm{P}}_{ }\times {\mathrm{T}}_{ }\times {\mathrm{R}}_{ }}{100}$

Now,

P₁  = Rs 4000

T₁ = 3 Yrs

R₁ = 18%

The simple interest for the amount borrowed by her at 18% p.a. = $\frac{4000\times 3\times 18}{100}$

S.I. at 18% p.a. = Rs 2160 

The remaining amount = 12000$-$4000 = Rs 8000 

Now,

P₂ = Rs 8000

T₂ = 3 Yrs

R₂ = 15%

The simple interest for the amount borrowed by her at 15% p.a. = $\frac{8000\times 3\times 15}{100}$

S.I. at 15% p.a. = Rs 3600

Total Interest = S.I. at 18% p.a. + S.I. at 15% p.a. = Rs 2160 + Rs 3600 = Rs 5760 

Hence, the correct answer is Rs 5760.