Answer:
.2
Step-by-step explanation:
When you do 64/80 you get 0.8 which means that the percent decrease is .2
We are given the first term and the common ratio, this means they belong to a geometric series.
For the given series:

Each term of the geometric series is obtained by multiplying the previous term by common ratio.
So the next terms will be:
-4.5, -6.75, -10.125, -15.1875, -22.78125
The general formula for the G.P would be:

On plotting the series, the result will be like this:
Answer: 53.4 cm
Step-by-step explanation:
The formula for circumference is C=2πr or C=πd. These equations are the same. Since the problem gives us the diameter, we'll go ahead and use C=πd.
C=(3.14)(17)
C=53.38=53.4 cm