Answer: 6.93 years
Step-by-step explanation:
Given
Rate of interest is 
Future value is given by 
For the investment to double itself, i.e. 

It takes around 6.93 years to double the investment.
Answer:
Step-by-step explanation:
Use the formula Sum = (a + L)*n/2
The tricky part is n. That's the number of terms between 1 and 99 inclusive.
n = 99 -1 + 1 = 99
n = 99
a = 1
L = 99
Sum = (1 + 99)*99 / 2
Sum = (100)*99/2
Sum = 4950
1/2 + 4x = 5x - 5/6
1/2 = x - 5/6
3/6 = x - 5/6
x = 8/6
The area of the circle is 1260.25 π m^2