Question

a=5 d=2.5 an=7.5 find n??

Answer 1

We have, a = 5, d = 2.5, an = 10
=>          an = a + (n-1)d
=>          10 = 5 + (n-1)2.5
=>          10 = 5 + 2.5n - 2.5
=>          10 = 2.5n + 2.5
=>       2.5n = 7.5
=>            n = 7.5/2.5 = 3

Answer 2

a=5
d=2.5
an=7.5
a+ (n-1)d =7.5
5+ (n-1)2.5 =7.5
2.5(n-1) =7.5-5
n-1= 2.5/2.5
n= 1+1
n= 2