Question: 11 -

A certain sum doubles in 7 years at simple interest. The same sum under the same interest rate will become 4 times in how many years.

14

10

28

21

Answer:

21

Solution:

Interest = Principal

Rate = (Interest×100)/(Principal × Time)

= 100/7% per annum

Case II,

Interest = 3 × Principal

Time = (Interest×100)/(Principal × Time)

= 3×1001007 = 3 × 7 = 21 years

Question: 12 -

Ram deposited a certain sum of money in a company at 12% per annum simple interest for 4 years and deposited equal amount in fixed deposit in a bank for 5 years at 15% per annum simple interest. If the difference in the interest from two sources is Rs.1350, then the sum deposited in each case is :

Rs.6500

Rs.5000

Rs.4000

Rs.3000

Answer:

Rs.5000

Solution:

Here, P₁=Rs.P, R₁=12%, T₁ = 4 years

P₂=Rs.P, R₂=15%, T₂ = 5 years

S.I. = Rs. 1350

S.I.= (P2xR2xT2−P1xR1xT1)100

1350 = (P×15×5−P×12×4)/100

135000 = 75 P - 48P

135000 = 75 P

P = Rs. 5000

Question: 13 -

A sum amounts to double in 8 years by simple interest. Then the rate of simple interest per annum is

15%

12.5%

20%

10%

Answer:

12.5%

Solution:

Principal = Rs.x

Amount = Rs.2x

Interest = 2x - x = Rs.x

Rate = SI×100/Principal × Time

= x×100/x×8=252 = 12.5 % per annum

OR

R % = ((n−1)/T)×100%

= (2−1)/8×100% = 12.5%

Question: 14 -

A sum of money at simple interest trebles itself in 15 years. It will become 5 times of itself in

30 years

25 years

40 years

36 years

Answer:

30 years

Solution:

R = ((3−1)/15)×100%

= (2/15)×100%

= (2/3)×20%=403%

T = (n−1)/R Years

= (5−1)/(40/3)×100 = 30 years.

Question: 15 -

The rate of simple interest for which a sum of money becomes 5 times of itself in 8 years is :

50%

40%

55%

30%

Answer:

50%

Solution:

Principal = Rs.x (let)

Amount = Rs.5x

Interest = Rs.(5x - x) = Rs.4x

Rate = S.I.×100/Principal × Time

= (4x×100)/(x×8) = 50% per annum