Find the fifth term of a geometric sequence for which a3 = 20 and r = 2.

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.