THANKS FOR THE ANSWER

Question

BUT I DON'T KNOW HOW TO WRITE THE PROBLEM
TO ARRIVE TO THAT ANSWER

Answer 1

I assume you mean the Santa page numbers?

no. of 1-digit pages = 9
no. of 2-digit pages = 90

so,

1*9 + 2*90 + 3n = 690
3n = 501
n = 167

9+90+167 = 266

Hmmm. I don't get 245. Check my math.