local a = {}
for i = 1, 20 do
    a[i] = {}
    for j = 1, 20 do
        a[i][j] = math.random(1, 20)
    end
end

for i = 1, 20 do
    for j = 1, 20 do
        io.write(string.format("%2d ", a[i][j]))
        if a[i][j] == 20 then
            print()
            break 2  -- break from outer loop
        end
        if j % 10 == 0 then print() end
    end
end