The coefficient of x6 in [(1+x)6+(1+x)7+....+(1+x)15] is
Find the coefficient of x15 in (x3+x6+x9+...........)(x2+x4+x6+...........)(x+x2+x3+..........)
Coefficient of x6 in ((1+x)(1+x2)2(1+x3)3....(1+xn)n) is