aritmetic progression

Common difference between terms is  5       (8-3 = 5      13-8 = 5 etc)

this is of the form    3 + 5 (n-1)    

                                3 + 5n -5 = 5n-2         where n= number of the term

           20th term       5(20)-2 = 98

Good idea.....!!!

The last term  is   253

And the common difference in terms is 5

So......the 20th term from the last  =

253 - 5(n - 1) =

253 - 5(20 - 1)  =  

253 - 5(19)  =

158

I suppose that you're technically correct......the common difference is -5 [ we are subtracting 5 each time ]

Chris pointed out to me that I mis-read your question.....

I will continue the way I started

we have 5n-2  gives the nth term

253 = 5n-2

255 = 5n

yields n = 51st term for 253      20 terms before this would be 31st term

5(31)-2 = 153   = 20 terms before 253

I think Chris answer may be the 19th before 253

 if you use his equation    the tem that is ONE before would be 253 - 5(1-1) = 253    which would be incorrect.

OK.....thanks, EP....