a/
uses crt;
var n,x,nx: word;
begin
clrscr;
write ('nhap n: '); readln (n):
write ('nhap x: '): readln (x);
nx:=n*x;
writeln ('nx la: ',nx);
readln;
end.
b/
uses crt;
var i,n,j: longint;
s: real;
begin
clrscr;
s:=1;
write ('nhap n: '); readln (n);
i:=20;
repeat
j:=i+2;
s:=s+1/j;
i:=i+10;
until (1/j<2*10-4);
writeln ('tong la: ',s:1:2);
readln;
end.