Question 1039797
Go to google.com and type future value to understand what it is. Also learn about future value calculator to see how it works
The present value is 20
The rate is .035
The period is 15
Here is the formula
F=p(1+r)^n 
P=20
R=.o35
N=15 
Using calculator we calculate f which is the future value
F is around $33.50

If it is doubled then
40=20(1.035)^n 
Here we have to calculate n which is the number of years. Dividing both sides by 20 we get
2=(1.035)^n
Using log
Log2 = log (1.035)^n
Log2=nlog (1.035)
We use calculator to find the two logs and we get n=20.
This means after 20 years the coin would worth $40
You have to learn how log works