The coefficient of x2y3in the expansion of (1−x+y)20is

The general term in the expansion of (1−x+y)20is 20!r!s!t!1r(−x)s(y)t, where r + s + t = 20For x2y3, we have the term 20!15!2!3!115(−x)2(y)3Hence, the coefficient of x2y3 is 20!15!2!3!