Answer:
The fith number is 80.
Step-by-step explanation:
A geometric sequence is a group of numbers that are related to it's neighbours by a product of a constant ratio. In order to calculate a certain number in such sequences we can use the formula:
an = a1*q^(n-1)
Where "n" is the position of the number we wish to know and q is the ratio. To use the formula we need to know the first number and we were given the third, so we first need to find that. We have:
a3 = a1*q^(3 - 1)
20 = a1*2^(2)
a1*4 = 20
a1 = 20/4 = 5
So the 5th number is:
a5 = a1*q^(5-1)
a5 = 5*2^(4)
a5 = 5*16 = 80
The fith number is 80.
