To solve the word problem:
- There are 6 days until Richard's concert.
- Susie's concert is 7 days after Richard's.
To find out how many days there are until Susie's concert, you would add the 6 days until Richard's concert to the 7 days after that until Susie's concert:
\( 6 + 7 = 13 \)
So, the correct number sentence that matches this word problem is:
6 + 7 = 13.