Decade Duel
Which is bigger: 65 or 49? Careful — the ones digit lies.
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.
- 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.
What this practises
Comparing two-digit numbers, including the pairs designed to trick you.
Rounds cycle through the four specific ways two-digit comparison goes wrong: tens and ones agreeing, tens and ones disagreeing, a shared tens digit, and the same two digits reversed. Practising the traps deliberately is more useful than practising random pairs.
- The underlying idea
- Place-value comparison, and the specific traps that defeat digit-by-digit reading.
- Where this sits in Illustrative Mathematics
- 2nd grade Unit 5: Numbers to 1,000
- Sources
- Holloway et al. (2009) · Institute of Education Sciences / What Works Clearinghouse (2021) all references →
Related measured skill: MagPI · Number comparison. What this means