#1**+1 **

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

ElectricPavlov
Mar 22, 2017

#3**+1 **

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

CPhill
Mar 22, 2017

#5**+1 **

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

CPhill
Mar 22, 2017

#6**+1 **

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.

ElectricPavlov
Mar 22, 2017