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when interest is reckoned using compound interest, interest being compounded annually. The difference in the simple interest and compound interest for two years is on account of the interest paid on the first year's interest  Hence 12% of simple interest = 90 => simple interest =90/0.12 =750.

As the simple interest for a year = 750 @ 12% p.a., the principal =750/0.12 = Rs.6250.

If the principal is 6250, then the amount outstanding at the end of 3 years = 6250 + 3(simple interest on 6250) + 3 (interest on simple interest) + 1 (interest on interest on interest) = 6250 +3(750) + 3(90) + 1(10.80) = 8780.80.

5% is the rate of interest. 20% of the interest amount is paid as tax.

i.e  80% of the interest amount stays back.

 if we compute the rate of interest as 80% of 5% = 4% p.a., we will get the same value.

The interest accrued for 3 years in compound interest = 3 x simple interest on principal + 3 x interest on simple interest + 1 x interest on interest on interest.

= 3 x (200) + 3 x (8) + 1 x 0.32 =600 + 24 + 0.32 = 624.32

The amount at the end of 3 years = 5000 + 624.32 = 5624.32

The population grew from 3600 to 4800 in 3 years. That is a growth of 1200 on 3600 during three year span.

Therefore, the rate of growth for three years has been constant.

The rate of growth during the next three years will also be the same.

Therefore, the population will grow from 4800 by 4800*13= 1600

Hence, the population three years from now will be 4800 + 1600 = 6400

The usual way to find the compound interest is given by the formula A = .p(1+r/100)^n

In this formula,

A is the amount at the end of the period of investment

P is the principal that is invested

r is the rate of interest in % p.a

And n is the number of years for which the principal has been invested.

In this case, it would turn out to be A =1500(1+20/100)^3

= 2592.

Let the sum be Rs. P.

Then,[p(1+10/100)2-p]=525

Sum =Rs.2500

S.I.= Rs.(2500*5*4)/100

= Rs. 500

[15000*(1+r/100)2-15000]-(15000*r*2)/100=96

Rate=8%

S.I. = Rs.(1200*10*1)/100=rs.120

C.I. =rs[1200*(1+5/100)2-1200]=rs.123

Difference = Rs.(123-120) =Rs.3

Sum = Rs.(50*100)/2*5=Rs.500

Amount=Rs.[500*(1+5/100)2]

= Rs. 551.25

 C.I = Rs.(551.25-500)= Rs.51.25