If you want to calculate on time go to tempotoets.nl
Also try how good you are at mental arithmetic!
Here you learn arithmetic error-free for FREE
It won't be on time, but when you're done, click "DONE" (duh) and you'll be shown how many you got right and how many wrong.
You will also be shown which sums you got wrong.
You will also be shown which sums you got wrong.