If the average interest is 8.13% then what percentage must he got for the remainder money ?

Question

A man lends Rs 10000 in four parts. If he gets 8% on Rs 2000, 7.5% on Rs 4000 and 8.5% on Rs 1400.
If the average interest is 8.13% then what percentage must he got for the remainder money ?

Answer 1

Total money=M=10,000
M1+M2+M3+M4=M
2000+4000+1400+M4=10000
M4=2600

(M1*R1)+(M2*R2)+(M3*R3)=(M4*R4)=M*R
(2000*8%)+(4000*7.50%)+(1400*8.50%)+(2600*R4%)=10000*8.13%
160+300+119+(2600*R4%)=813
(2600*R4%)=234
R4%=234/2600
R4=9

Ans is 9%