Mistake Library
This page helps students learn from the most common errors. Topic sections unlock as the course is completed.
Numbers
Numbers and Place Value
- Rounding from the wrong digit.
- Thinking 401 is bigger than 4,010 because 401 starts with 4.
- Forgetting that negative numbers get smaller as they move further from zero.
Addition
Addition and Subtraction
- Not lining up digits properly.
- Forgetting to carry or exchange.
- Subtracting the smaller digit from the larger digit in each column instead of following the top number.
Multiplication
Multiplication and Division
- Mixing up factors and multiples.
- Forgetting remainders in division.
- Not checking division by multiplying back.
Fractions
Fractions
- Adding denominators.
- Comparing fractions without a common denominator.
- Forgetting to simplify the final answer.
Decimals
Decimals
- Ignoring decimal point alignment.
- Thinking 0.7 is smaller than 0.65 because 7 is one digit.
- Removing zeros in a way that changes place value.
Percentages
Percentages
- Treating 20% as 20 instead of 0.20.
- Forgetting that 100% is the whole amount.
- Not checking whether a percentage increase should make the answer bigger.
Ratio
Ratio and Proportion
- Dividing by one ratio number instead of total parts.
- Changing only one side of a ratio.
- Forgetting to check the total.
Algebra
Algebra
- Combining unlike terms such as x and x squared.
- Doing an operation to only one side.
- Forgetting to check the answer in the original equation.
Geometry
Geometry
- Mixing up area and perimeter.
- Forgetting squared units for area.
- Not using angle facts such as 180 degrees on a line.
Units
Measurement and Units
- Multiplying when you should divide.
- Mixing units in the same calculation.
- Leaving the unit off the answer.
Statistics
Statistics and Data
- Finding the mean when asked for the median.
- Forgetting to order data for the median.
- Using the biggest number as the range instead of highest minus lowest.
Powers
Powers and Roots
- Thinking 5 squared means 5 x 2.
- Forgetting that roots undo powers.
- Rushing powers without expanding them.
Exam
Exam Trainer
- Answering a different question from the one asked.
- Writing only the final answer without method.
- Forgetting units or final statements.