Decade Duel

For grown-ups

• How long: 5 to 10 minutes is plenty. Short and frequent beats long and occasional — the early-numeracy trials with the largest effects ran eight weeks or less.
• The strategy to reinforce: “Tens first” — Compare the tens digits. Only look at the ones if the tens are the same. If they stall, remind them of this rather than giving the answer.
• When they get one wrong: the page shows the working. Read it together rather than moving straight on. Feedback that explains is worth several times more than feedback that only marks right or wrong, and that gap is wider in maths than in any other subject.
• It adjusts as you go: this one gets harder while they are getting them right and easier if they are not, aiming to keep them succeeding about four times in five — hard enough to be worth doing, not hard enough to stop being fun. The score is how deep they got rather than how many they answered, because the difficulty moved. Nothing is stored — it starts fresh every time the page loads.
• On the timer: it is off unless you turn it on, and it is never needed. Use it only once the skill is comfortable, never to introduce it.
• Print it too: doing the same problems on paper a few days later is worth more than doing twice as many today.