Question

IN a.p a=13,t15=55,d=?

Answer 1

if a=13 and t15=55 then
a+14×d=55
= 14d=55-13
= d=42/14
= d=21/7
= d=3

Answer 2

a = 13

T15 = 55

=> T15 = a + (n-1)d

=> 55 = 13 + 14d

=> 14d = 42

=> $\fbox{d = 3}$