# Simple and Compound Interest Tricks – 2

Read this post carefully and note down all the formulas in a piece of paper for quick revision.

**Q. 1) If a sum of money becomes 3 times itself in 20 years at simple interest. What is the rate of interest?**

In such questions apply the direct formula-

Rate of interest = [100*(Multiple factor – 1)]/T

So R = 100*(3 – 1)/20

**Answer : 10%**

** Note : **With this formula you can find Rate if Time is given and Time if rate is given.

**Q. 2)**

In such questions, just write this line :

**1st part : 2nd part : 3rd part = ****1/(100+T1 * r) : 1/(100+T2 * r) : 1/(100+T3 * r)**

= 1/(100+2 * 5) : 1/(100+3 * 5) : 1/(100+4*5)

= 1/110 : 1/115 : 1/120

= 23*24 : 22*24 : 23*22

Hence 1st part = (23*24)/ (23*24 + 22*24 + 23*22) * 2379

**Answer : 828**

**Note : **Surprisingly, such questions when asked mostly have this same data, i.e., R=5% and T1, T2, T3 = 2, 3, 4 years, respectively. Only the Principal is changed. So it would be wise if you can just mug this line :

**1st part : 2nd part : 3rd part = 23*24 : 22*24 : 23*22
Based on the above line, you would be able to solve such questions in a jiffy. **

__But note that it will only work if the question is on Simple Interest__. Like the below question appeared in SSC CGL Tier 2-Here the data is same**. **i.e., R=5% and T1, T2, T3 = 2, 3, 4 years, respectively. So we will write directly –

**1st part : 2nd part : 3rd part = 23*24 : 22*24 : 23*22**

A received** = **(23*24)/ (23*24 + 22*24 + 23*22) * 7930

**Answer : Rs. 2760**

**Q. 5) **If a certain sum of money P lent out for a certain time T amounts to P_{1} at R_{1}% per annum and to P_{2} at R_{2}% per annum, then

**The above formula is for calculating the Time, if the question asks the rate, then just interchange the rate and time. Hence the formula will become**

**R = (P**T

_{1 – }P_{2)*100/}P_{2}

_{1 – }**P**T

_{1}

_{2}

Apply the formula:

R = (650-600)*100/600*6 – 650*4

R = 5%

**Alternative Method :**

**Answer: 5%**

**For CI, the formula is different**

__Difference between CI and SI__

**If t=3 years then difference between CI and SI can be given by two formulas-**

**In all the above formulas we have assumed that the interest is compounded annually**

**Let us solve some CGL questions**

1.44P = P(1 + r/100)^2

**Answer : (D)**

P(1.05^3 – 1) – 0.15P = 244

P = Rs. 32000

**Answer : (C)**

**Answer : (D)**

A = P(1+r/100)^t

**Answer : (B)**

2.5 = 10R – 10r

R – r = 0.25

**Answer : (D)**

Total CI = 0.05P + 0.0525P = 0.1025P

**Answer : (C)**

**Note : You can solve this question by the formula A = P(1+r/100)^t as well**

**Answer : (D)**

**Answer : (A)**

**Answer : (C)**

= [(1587/1200)^1/2 – 1]*100

= 15%

**Answer : (B)**

**So this is the end of CI and SI series. If you have any doubt in this topic, please drop a comment…**