A can do a piece of work in 15 days working 6 hours per day and B can do it in 10 days working 5 hours per day. How long (in days) will they take to do it, working together for 9 hours per day?
1
\(6 \frac{4}{7}\)
2
\(5 \frac{4}{7}\)
3
\(3 \frac{4}{7}\)
4
\(2 \frac{4}{7}\)