UPSC CDS Math Number System Questions

The numbers between 500 and 1000 which are divisible by 13 are 507, 520, 533,..., 988.
These numbers are in AP with first term, a = 507 and common difference, d = 13
Let 988 be the nth term of the AP.
∴507+(n−1)×13=988
⇒13(n−1)=481
⇒n−1=37
⇒n=38