Given expansion is (1+3x+2x2+x3)20 =((1+x)3)20 =(1+x)60 ................(i) In the expansion of (a+b)^n,(r+1)th term given by tr+1=nCrar.bn−r or, tr+1−nCran−r.br ∴ 21st term in Eq.(i) will contain the coefficient of x20and it is given by t21=60C20x20,140=60C20x20 ∴ Coefficient of x20 in (1+x)60 is 60C20 or 60C40.