10 Important Compound Interest Tricks

Question 1. 

Example: P=20000 Rs. , r= 10%. t= 2 years, A=?
Solution:

Question 2.

Example: What will be the compound interest on Rs. 12000 after 3 years at the rate of 10% per annum?
Solution:

Question 3.

 If a sum of money becomes n1 times itself at compound interest in t1 years then after t2 sum will be:

Example: A sum of money becomes double itself in 5 years at compound interest. In how many years will it become 8 times of itself?
Solution:

Question 4. 

Example: 

Solution:

Question 5.

Calculation of instalment for 2 years:
Convert r% into fraction (a/b)

Example: A man borrowed Rs.8,400 at 10% p.a. CI. He pays equal annual instalment and clear off his debts in 2 yrs. Find the instalment?
Solution:

Question 6. 

Calculation of instalment for 3 years:
Convert r% into fraction (a/b)

Example: A man borrowed Rs.397950 at 6% p.a. CI. He pays equal annual instalment and clear off his debts in 3 yrs. Find the instalment.
Solution:

Question 7. 

If the difference between C.I. and S.I. for 2 years is given, then Principal:


Example:  If the difference between S.I. and C.I. for 2 years on certain money is 4 Rs. The rate of interest is 8% per annum then find the money.
Solution:

Question 8. 

If difference between C.I. and S.I. for 3 years is given, then Principal:


Example:  If the difference between S.I. and C.I. for 3 years on certain money is 31 Rs. The rate of interest is 10% per annum then find the money.
Solution:

Question 9. 

A sum of money invested at compound interest amounts to 800Rs. in 3 years and Rs. 882 in 5 years. What is the rate of interest?
Solution:

Question 10.

When r% is given p.a. and CI has to be calculated half-yearly or quarterly basis:

Yearly	factors	r% (per annum)	Time (n years)
Half yearly	6 months= (6/12)=1/2	Factor ×r%= r/2%	2n
Quarterly	3 months= (3/12)=1/4	1/4 ×r% = r/4%	4n
9 months	9 months= (9/12)=3/4	3/4× r%= 3r/4%	4n/3
8 months	8 months= (8/12)= 2/3	2/3× r%= 2r/3%	3n/2