To show that this is a geometric series, and to find r,  
we divide the 2nd term by the 1st term.  Then we divide 
the 3rd term by the 2nd term, and if those two numbers
are the same then it is a geometric series and r = what
we got when we divided those.

15÷75 =  = 

3÷15 =  = 

They are the same so it is a geometric series with r = .

It is an infinite series with sum given by the formula

 =  =  =  = ÷ = × =  = 93.75

Edwin