(a)

The formula is  .

 The principal value is

The total amount .

The rate of interest is .

Substitute all the values.

Therefore,the time period is years .

(b)

 The principal value is

The total amount .

Time period is

Find the rate of interest .

Substitute all the values in .

The rate of interest is .

(c)

Derive the formula  .

Compound interest compounded continuously .

.

Rewrite the equation.

.

Apply natural logarithm on each side.

.

.

Therefore,time period is .

(a) The time period is years .

(b) The rate of interest is .

(c)The formula for time period is .