Principal:
The money borrowed or lent out for a certain period is called the principal or the sum.
Interest:
Extra money paid for using other's money is called interest.
Simple Interest (S.I.):
If the interest on a sum borrowed for certain period is reckoned uniformly, then it is called simple interest.
Let Principal = P, Rate = R% per annum (p.a.) and Time = T years.
Then,
Simple Interest (SI) = P x R x T/100
Compound Interest (C.I.)
Let Principal = P, Rate = R% per annum, Time = n years
When interest is compound Annually:
Amount = P(1+R/100)^{n}
When interest is compounded Halfyearly:
Amount = P[1+(R/2)/100]^{2n}
When interest is compounded Quarterly:
Amount = P[1+(R/4)/100]^{4n}
1. 

I = PNR/100
Here, P = 1200
N = 4
R = 5%
I = (1200 x 4 x 5)/100 = ₹240.
I = ₹240.
2. 

I = PNR/100
I = (10000 x 2 x 10)/100
I = 2000.
3. 

Interest for the first year
= (1000 x 1 x 10)/100 = 100
Principal for second year
= 1000 + 100 = 1100
Interest for the second year = (1100 x 1 x 10)/100 = 110
Hence, Total interest for the two years = 100 + 110 = ₹210.
4. 

I = Amount  Principal = 2200  2000 = ₹200.
I = PNR/100
200 = (2000 x 2 x R)/100 = 5%
5. 

A = P(1 + R/100)^N
24200 = 20000(1 + R/100)^2
1.21 = (1 + R/100)^2
R = 10%
6. 

Interest for first six months = (200 x 1/2 x 10)/100 = ₹10
Principal for next six months = 200 + 10 = ₹210
Interest for next six months = (210 x 1/2 x 10)/100 = ₹10.5
Total interest = 10 + 10.5 = ₹20.5
7. 

Let principal = P.
Then, S.I. = P
And T = 16 years.
R = (100 x P)/(P x 16)% = 25/4% p.a.
8. 

Let the sum lent at 8% be ₹ X and that at 6% be ₹ (1550  X).
[8 x 1 x X]/100 + [(1550  X) x 6 x 1]/100 = 106
8X + 9300  6X = 10600
2X = 1300
X = 650
Money lent at 8% = ₹650.
Money lent at 6% = ₹(1550  650) = ₹900.
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