# math

posted by
**Anonymous**
.

How many strings of length 13 can be formed from 9 1's, two ('s and two )'s such that the substring () never occurs, and for k=1,2, we see k('s before k)'s?

posted by
**Anonymous**
.

How many strings of length 13 can be formed from 9 1's, two ('s and two )'s such that the substring () never occurs, and for k=1,2, we see k('s before k)'s?