18.90? I believe would be the answer . Let's see if someone else responds
Arithmetic sequence has a common difference between the terms.
for 1,4,7,10,...
4-1 = 3
7-4 = 3
10-7 = 3
so we do see a common difference between the terms.
Hence, True
Well, let's assume March 1st is Sunday. April 1st is Wednesday, May 1st is Saturday, June 1st is Tuesday, July 1st is Thursday, August 1st is Friday, September 1st is Sunday. So, her birthday must be in September.