If 1=5, 2=10, 3=20, 4=40 , then we have a geometric series which is:

`5, 10, 20 , 40, ....`

And its common ratio is:

`r =10/5=20/10=40/20 = 2`

Using the formula,

`a_n=a_1r^(n-1)`

the fifth term is:

`a_5=5*2^(5-1)=5*2^4=5*16=80`

**Hence, 5 = 80.**