Question

Nigel is participating in a read-a-thon. The number of pages he reads each night follows a geometric sequence. On the second day of the read-a-thon, Nigel read 8 pages. On the fifth day of the read-a-thon, he read 64 pages. Write an explicit formula to represent this scenario.
Explicit Formula: an=a1 * r^(n-1)
Answer: an=4(2)^n-1

I just don't know how to get to the answer with the information given.
Any ideas?
Can someone delete the question, please? Thank You.
a r^0 ar^1 ar^2 ar^3 ar^4 ar^5

a ar^1=8 ...... ar^4 = 64
so
ar^4/ar^1 = 64/8 = 8
r^3 = 8
r = 2 which we could have guessed when we saw 8 and 64
if ar = 8 and r = 2 then a =4
so
4 *2^(n-1)

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