9 girls working 7 hours a day can complete a piece of work in 15 day. In how many days can 6 girls working for 9 hours a day, complete the same piece of work?

Let the number of days be 'p' 

As the work is same, we know that

$M1 x D1 x H1 = M2 x D2 x H2$

Where M = Men, D = Days, H = Hours per day 

Here M1 = 9, D1 = 15, H1 = 7 

M2 = 6, D2 = p, H2 = 9 

=> 9 x 15 x 7 = 6 x p x 9 

=> p = 35/2 = 17.5 days.