6 boys and 6 girls are sitting in a row . Then the no. of ways that boys and girls sits alternatively

my solution::

Using gap method::

here X = denote boys and _ = denote girl

Then X _ X _ X _ X _ X _ X _

first we can arrange boys , which can be done in 6! ways

now arrange girls in 7 gaps ,

for that first we select 6 place out of 7 which can be done in C (7,6) ways and now arrange these 6 girls

Which can we done in C(7,6) * 6! = 7!

So total no. of ways is = 6! * 7!

but answer is = 2*6! *6!

so where i have done mistake

Thanks