(1+x+x^5)^5=[ (1+x)+x^5 ]^5=(1+x)^5 + 5(1+x)^4.x^5 +10.(1+x)^3.x^10 +...+x^25coef of x^6 : (1+x+x^5)^5=coef of x^6 : 5(1+x)^4.x^5=5 (4C1)=20
