Vishwas borrowed a total amount of Rs. 30000, part of it on simple interest rate of 12 p.c.p.a. and remaining on simple interest rate of 10 p.c.p.a. If at the end of 2 year she paid in all Rs. 36840 to settle the loan amount, what was the amount borrowed at 12 p.c.p.a ?

Explanation:-

Let the sum borrowed at 12% p.a. be Rs. x
and that borrowed at 10% p.a. be Rs. (30000 - x)
S.I. at the end of 2 years
= Rs. (36480 - 30000)
= Rs. 6480
$$\eqalign{ & \therefore \left( {\frac{{x \times 12 \times 2}}{{100}}} \right) + \left[ {\frac{{\left( {30000 - x} \right) \times 10 \times 2}}{{100}}} \right] = 6480 \cr & \Leftrightarrow 24x + 600000 - 20x = 648000 \cr & \Leftrightarrow 4x = 48000 \cr & \Leftrightarrow x = 12000. \cr}$$