How to Make Fewer Mental Math Mistakes: Tips for Finding Where You Went Wrong

Look at the difference before “one more try”

Mental math mistakes actually fall into just a few patterns. If you move on to the next problem without checking the cause when your answer is wrong, you’ll repeat the same kind of mistake again and again. If you know which patterns you tend to fall into, it becomes clear what to focus on in practice.

The clue is the difference between your answer and the correct one. Just looking at what kind of number the difference is lets you deduce, with a fair degree of confidence, what happened and where.

Mistake patterns you can read from the difference

This site’s flash mental math tool does part of this deduction automatically when you check your answer. If your answer matches a running total, it tells you it equals the total up to that number; if not, it shows the difference and adds that, if the difference equals one of the numbers shown, you may have counted it twice (or missed it) or mixed up adding and subtracting, and if it’s a multiple of 10, you may have miscounted a carry or borrow.

Steps for pinpointing the number you got wrong with running totals

1. 1

   Look at the table of numbers and running totals

   After checking your answer, you get a list of the total from the first number up to each number. If your answer equals one of these running totals, you failed to add from the next number onward.

2. 2

   Look for a number that matches the difference

   If a number equal to the difference was shown, you either missed it or counted it twice. Seeing which position it was in tells you where you tend to slip (the start, the middle or the end).

3. 3

   Solve the same problem again

   Press “Same problem again” and the same sequence of numbers is shown at the same speed. Redo it with the number you got wrong in mind and check whether you get it right this time.

4. 4

   Note down the pattern

   Writing down the tendency over several problems, such as “I tend to miss the last number” or “I go wrong carrying into the tens”, gives your practice a direction.

How to fix each pattern

• Lots of missed numbers: a sign the speed doesn’t suit you. Lengthen the seconds per number by 20%. If it happens at a particular position (the first or last), focus on watching the center of the screen from the countdown onward, and on staying alert until the last number.
• Counting twice: you’re adding a number you’ve already added. Mark “done” during the blank gap, and keep only one running total in your head.
• Miscounted carries and borrows: complement moves may not be automatic yet. Slow the conditions down and practice until combinations that use complements (adding 7, subtracting 8 and so on) come without hesitation. Complements are explained in tips for flash mental math with addition and subtraction.
• Mixing up adding and subtracting: build the habit of looking at the sign before the number. In the tool, numbers to subtract are in red and the beep is lower, so use color and sound as cues.
• Stopping partway (matches a running total): your concentration is breaking, or the count is too high for your current level. Lower the count and practice without changing the seconds per number.

Practice habits that reduce mistakes

Carrying on at fast conditions while accuracy is low builds a habit of giving vague answers. If you keep getting 5 or fewer out of 10 right, going back to the previous conditions to rebuild accuracy gets you further faster in the end. For how to raise conditions, see how to practice flash mental math, and for speed guides, the seconds-per-number chart.

• Always end a practice session on conditions you can get right
• Redo at least one problem you got wrong as the same problem
• Look at your history, and when accuracy starts to drop, take it as a sign of tiredness and stop
• When children practice, don’t blame them for the number they missed; look together for where things went off track

The tool’s history (the last 30 results) and level are saved only in this device’s browser. If your family shares one device, using separate browser profiles keeps the records from getting mixed up.

FAQ

Is there a way to check where I went wrong in flash mental math?

Look at the difference between your answer and the correct one. If it equals one of the numbers shown, you probably missed it or counted it twice; if it’s a multiple of 10, a miscounted carry is likely. When checking answers, the flash mental math tool shows a table of running totals and the likely causes based on the difference.

Why do I miss so many numbers in mental math?

In most cases, the speed doesn’t suit your current level. Lengthening the seconds per number by about 20% often helps. If you only miss the first or last numbers, focus on watching the center of the screen before it starts and staying alert to the end.

Why is my answer off by exactly 10 or 100?

It happens when you forget a carry or borrow once, or count one too many. Complement moves may not be automatic yet, so practice combinations that use complements at slower conditions.

Can I solve the same problem again?

In the flash mental math tool, pressing “Same problem again” shows the same sequence of numbers again at the same conditions. Redoing it with the number you got wrong in mind helps confirm the cause.