Get comfortable with fractions and decimals.
Learn a method without a timer, then use it in a mixed round.
Integer arithmetic
For whole-number addition, subtraction, multiplication and division, use the Zetamac practice guide. Its rounding, splitting and missing-factor methods apply here too; 80 in 8 uses its own question ranges and can include negative operands.
The decimal and fraction families in 80 in 8 use addition and subtraction. The examples below are untimed and save nothing.
Fractions: use a common denominator
Simplify the displayed operands first when it helps. Find the smallest denominator both fractions can share, then add their numerators.
- 12/6 + 1/4 = 1/3 + 1/4Reduce 2/6 by dividing both parts by 2.
- 21/3 + 1/4 = 4/12 + 3/12The least common multiple of 3 and 4 is 12. Multiply both parts of each fraction by the same number.
- 34/12 + 3/12 = 7/12Add the numerators and keep the denominator. The result is already reduced.
Avoid this mistake. Do not add the denominators. 1/3 + 1/4 is 7/12, not 2/7. Reduce the result before submitting.
Another useful method
If one denominator divides the other, use the larger one: 1/2 + 1/4 = 2/4 + 1/4 = 3/4. Learn common denominator pairs such as 3 and 6, 4 and 8, and 6 and 12.
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- 2/12 + 3/12 = 5/12.
- 5 and 12 have no common factor greater than 1.
1/6 + 1/4 = 5/12
Fractions: subtract and keep the sign
Use a common denominator for subtraction too. Compare the fractions to predict whether the answer will be positive or negative.
- 11/4 − 5/6A quarter is smaller than five sixths, so the result will be negative.
- 21/4 − 5/6 = 3/12 − 10/12Rewrite both fractions in twelfths.
- 3(3 − 10)/12 = -7/12Subtract in the original order. Put the minus sign before the numerator.
Avoid this mistake. Do not reverse the operands to avoid a negative result. A negative first operand stays negative: -1/3 − 1/6 = -2/6 − 1/6 = -1/2.
Another useful method
Check the sign first, then calculate the numerator and reduce. For equal denominators, subtract directly: 5/8 − 1/8 = 4/8 = 1/2.
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- 1/3 − 3/4 = 4/12 − 9/12.
- (4 − 9)/12 = -5/12.
1/3 − 3/4 = -5/12
Decimals: work in the same units
Match the decimal places before calculating. For tenths and hundredths, you can treat both operands as whole numbers of hundredths.
- 112.7 + 3.46 = 12.70 + 3.46A trailing zero does not change the value; both numbers now show hundredths.
- 21,270 + 346 = 1,616 hundredthsUse your integer addition method on matching units.
- 31,616 hundredths = 16.16Divide by 100 to restore the decimal point. Check: about 13 + 3 is 16.
Avoid this mistake. Align place values, not the last digits. 0.7 + 0.06 is 0.76, not 0.13. Restore the decimal point after working in hundredths.
Another useful method
Rounding also works with decimals: 8.95 + 2.7 = 9 + 2.7 − 0.05 = 11.65. With a negative first operand, compare magnitudes: -2.7 + 1.45 = -1.25.
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- 680 + 235 = 915 hundredths.
- 915 hundredths = 9.15.
6.8 + 2.35 = 9.15
Decimals: subtract, then check the scale
Match place values before subtracting. For nearby operands, counting up can be quicker than borrowing across several places.
- 110.2 − 3.75 = 10.20 − 3.75Write both operands in hundredths.
- 21,020 − 375 = 645 hundredthsSubtract 400 and add 25 back: 1,020 − 400 + 25 = 645.
- 3645 hundredths = 6.45Restore the decimal point. Check: 10.2 − 3.8 is about 6.4.
Avoid this mistake. Keep the subtraction order and check the sign. 3.75 − 10.2 = -6.45. Subtracting a positive number from a negative one makes it more negative.
Another useful method
For 8.03 − 7.98, count up: 7.98 to 8 is 0.02, then 8 to 8.03 is 0.03. The difference is 0.05. Choose the shorter reliable method.
Try It Yourself
Show method and answer
- 940 − 300 + 15 = 655 hundredths.
- 655 hundredths = 6.55.
9.4 − 2.85 = 6.55
Focus on one family, then mix it back in.
- 01 · Learn
Work slowly first
Try the examples, check the method and explain each adjustment. For fractions, make reducing the answer part of the calculation.
- 02 · Focus
Choose your question families
While idle on the game page, open Settings and keep only the families you want to practice. For fraction practice, begin with a smaller denominator cap, then increase it as you become comfortable. Save before starting.
- 03 · Mix
Return to a mixed round
Re-enable the other families or reset defaults before testing your mixed pace. Review incorrect answers, skips and slower correct solves in 80 in 8 stats, then choose one skill for next time.
Bring the methods into your round
The default is 80 questions in eight minutes: an average budget of six seconds per question to reach all 80. Use that as a pacing reference; some questions need more time than others.
Enter submits your answer, and incorrect answers advance too. Check the sign and answer format before submitting. Skip moves on without a point; review skipped questions afterward.
Answer formats and final checks
- Use a dot for decimals: 0.75. Trailing zeros do not change the value.
- Use a reduced fraction: 1/2, rather than 2/4. Put a negative sign before the numerator: -1/2.
- Write whole-number fraction results as integers: 1, rather than 2/2 or 1/1. Do not use mixed-number notation; write 5/4 rather than 1 1/4.
- Estimate the sign and size: adding two positive fractions cannot give a negative result, and 12.7 + 3.46 should be near 16.
Compare like-for-like practice
Compare finished rounds with matching duration, target and question settings. A fractions-only round trains a different skill from the default mixed test. Submitted-answer accuracy counts correct answers divided by submitted answers; skips and unanswered questions stay separate.