At what rate percent per annum will the simple interest on a sum of money be 2/5 of the amount in 10 years

Answer: Option B

Explanation:

Let sum be x and rate be r%
then, (x*r*15)/100 = 3x [important to note here is that simple interest will be 3x not 4x, beause 3x+x = 4x]

=> r = 20%

Answer: Option D

Explanation:

Let sum = x
Time = 10 years.
S.I = 2x /5, [as per question]
Rate =( (100 * 2x) / (x*5*10))%
=> Rate = 4%

Answer: Option D

Explanation:

We need to calculate the profit of B.
It will be,
SI on the rate B lends - SI on the rate B gets

$$\begin{aligned} \text{Gain of B}\\ &= \frac{3500\times11.5\times3}{100} - \frac{3500\times10\times3}{100}\\ = 157.50 \end{aligned}$$

Answer: Option C

Explanation:

SI for 3 year = 2600-2240 = 360
SI for 2 year 360/3 * 2 = 240
principal = 2240 - 240 = 2000

Answer: Option D

Explanation:

We need to calculate the total amount to be paid by him after 4 years, So it will be Principal + simple interest.

So,
$$\begin{aligned} => 500 + \frac{500*5*4}{100} => Rs. 600 \end{aligned}$$