@Carl Love Doesnt work. Im using Maple 7 version.

@Carl Love thank you can you send this program code please

@Kitonum I saw now. Thank you very much :)

@Kitonum  ty. i understand your solve but i need to do make with need 2 "for" loops.  This is my homework. I cant use seq command. I need to use 2 for loops. :/ 

@Kitonum ty but i need code. Not Mean(...). I need program. Like this

total:=0;

numb:=0;

for i from 1 to 3 do

for j from 1 to 5 do

N[i,j]:=20*i;

total:=total+N[i,j];

numb:=numb+i

end do;

end do;

evalf(total/numb);