Question

Find the 20th term from the last of the arithmetic progression 3,8,13...253

Answer 1

if we want to find value from last so we can write it from back..
253,248,243,......,8,3
now here first term is 253
and difference is -5..
a20=a+19d
253+19(-5)..
158...

hope you understand..
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Answer 2

3,8,13...........253

a=3

d=8-3=5

let us find which is 253 term

i.e. an=253

a+{n-1}d=253

3+{n-1 }*5=253

n-1=253-3/5

n-1=50

n=50+1

n=51

now,20^th term =51-19

                           =32

now, find a32

i.e.,

an=a+{n-1}d

a32= 3+{32-1}5

      =158

Hope it helps u.