In this problem
Let
y -----> the number of pennies
x-----> the number of days
In this problem we have a exponential function of the form
y=2^(x-1)
Verify
For x=1 day
y=2^(1-1)
y=2^0
y=1
For x=2
y=2^(2-1)
y=2^1
y=2
For x=3
y=2^(3-1)
y=2^(2)
y=4
therefore
This relationship is not an example of a constant rate
Is an example of a exponential function