первый
var
a : integer := 1;
s : integer;
begin
while a <= 50 do begin</p>
if a mod 2 = 0 then s += a;
a += 1;
end;
writeln('s = ', s);
end.
второй
var
a : integer := 1;
s : integer;
begin
repeat
if a mod 2 = 0 then s += a;
a += 1;
until a > 50;
writeln('s = ', s);
end.
третий
var
s : integer;
begin
for var a := 1 to 50 do if a mod 2 = 0 then s += a;
writeln('s = ', s);
end.