Question

anju invested Rs 12000 at 5 percent per annum compound interest . after how many years will her investment amount to Rs 13230 .

Answer 1

Principal = 12000  Rs



After 1 year he paid back Rs 4000




Let the rate of interest be x
Hence the principle after 1 year 12000

(1+x/100)-4000



= {12000(1+x/100)-4000 +920}




=> (1+x / 100)= {12000(1+x / 100)-4000 +920}



/{12000(1+x / 100)-4000}



 
=> (1+x/100)= {8920+120x } /{8000+120x}
 
=> x/100=   {8920+120x}/{8000+120x}   - 1
 
=> x/100 = 920/8000+120x
 
=> 120x2+8000r-92000 = 0
 
=> 3x2+200x-2300=0
               
=> 3x2 -30x+230x - 2300=0
 
=> 3x(x-10)230(x-10)=0
 
=> (3x + 230)(x -10)=0

.......



rate of interest =10%


*******************************************


initial principle = 12000




C.I for 1st year = 12000(1+10/100)-12000
                           =13200-12000=1200

-----------





C.I. for 2nd year =920



⚪ sum of principle and interest of both years


=. 12000+1200+920=14120





-------. paid back 4000





⚪⚪⚪ Hence amount of debt at the end of 2nd year = 14120-4000= 10120 Rs.